I think others have generally caught this, but I wanted to simplify the point: the apps on your phone are not controlling your home, a computer is. If you don't use Google's, then you'll need to provide one.

so can someone explain why a pretty robust dedicated device is necessary as a replacement?

The cloud is just someone else's computer, so when you cut the cord from the cloud, you gotta run your own server.

And you don't need to buy a (robust) device to run HA, just install it on a spare system and start playing with it. I started building mine about 1.5yrs ago when I bought a house and I think I only gave mine like 2 CPU and 8gb ram.

What actually happens when I turn on a smart switch in my home? Does that command have to be sent to a server somewhere to be processed?

Yes, you have to have something that accepts your commands and sends the action to the end device. Just like your Google home did.

What really has to be processed, and why can't a smartphone app do it?

Because that's not how things work. Your app has to talk to a server to send the commands, Google home has cloud servers and a local bridge. HA has an app that you can use to control your stuff, same as Google Home.

Smart Home apps are worthless without hardware required to connect the app to your home.

Proprietary products just use a simple app to manage and control devices

They have a dedicated set of servers your devices and app are connecting to, that's what home assistant is essentially replacing.

It's not just app > device, it's app > server > device.

My homeassistant is running on a pi2 with 2GB RAM. it doesn't need much.

But yes, it is a central place for processing and recording data, either from phone, imstalled electrical hardware or other devices.

in a nutshell

This is how the control and information exchange of smart devices work:

Phone App -> [Server] ... [Server] -> Smart Device and vice versa

There's no way around this concept.

Now, Google gives you the phone app and the (public) server part. but these only work with their servers and apps, keeping you locked in.

HA gives you the same, a server and an app, but allows you to keep the server private (access via vpn for public)

Also who guarantees that Google Home will be there in the next few years? HA will still keep running even if it ever gets abandoned.

Depending. But yes. A lot of the time they go the long way around, and head out to some server and back to your local network.

If the internet is down, or the vendor shits off the server, the device stops working.

Home assistant is attempting to localise everything, and get different vendors devices to work together.

Note, for your hass install, it can be installed on any server. Though I am using the green device

A service needs a server, not just a phone. Do you want to self host, or use the entire hardware installation in someone’s cloud data center?

You can run HA on lighter hardware, or use a VM or docker, but it needs something to host.

When I say “hey Siri open my garage door”, my phone does voice recognition, calls the Apple Home app, which calls my local Home Assistant server, which calls the RATGDO which opens the door.

The idea of Home Assistant is not to be a replacement for anything. It rather connects all things. It is a smart home control center, or hub.

Compare it to a Homematic, or maybe Aquara hub, etc. but still more feature rich and expandable with many more protocols and device categories.

Proprietary single switches etc. use only their own protocol.

Google Home is limited to a few protocols.

Yes, Google runs their servers in the cloud. If you host it yourself, it runs off a device in your house.

But you don't need to use their own hardware. You can run on whatever you like, and it probably doesn't actually need all four vCPUs.

Bluetooth can do it locally, but yes, for things on ZigBee or Z-Wave, it's gotta have an antenna hub. WiFi switches and lights most likely do "phone-home" to the cloud in some way (usually for color or brightness control via app, Govee especially loves this). The down side, other than the obvious privacy implications, is that if your ISP has an outage, so do your switches.

Home Assistant attempts to mitigate both the privacy and offline issues, while putting all of the different brands and hubs into one place.

My words.

When you read "Cloud" change the word in your head to "Someone else's computer"

So when I was using Tuya lightbulbs I was connecting Tuya's Computer to Googles Computer to turn my lights on.

Now I use Home Assistant, I'm connecting MY computer to Googles Computer to turn the lights on, since I'm asking Google to turn MY lights on.

I can just not use the Google Cloud and have a voice assistant in Home Assistant do it, then I'm just using my own computer.

Now then, you don't think that all the photos you've ever taken reside in an app on your phone do you, because if you do I've got some news for you....

It doesn't take much to run home assistant. I just have a raspberry pi going and it never fails. I plan to upgrade it to something more powerful in the future, but for now it is more than capable of running home assistant for me with no issues.

HA doesn't require 4/4/32, that's just the hardware the HA people sell. (which, given that your phone may be 8/16/128, is hardly "robust"). Generally, the Home Assistant crowd kind of target an audience that's probably already running some kind of home server, NAS, or router, and HA can probably be installed on that device.

Theoretically, there's no reason the HA server couldn't be installed on your phone, except then your smart home functions would only work while your phone is in the house and not sleeping. Kind of defeats the point of a lot of it, unless you're just thinking of smart home like "remote control for everything." Regardless, much smaller niche for an already-small market, and apparently not a priority for the dev team.

Home Assistant can be installed as a Docker container instead of going with the whole dedicated machine route.

While there are technologies that work like this (zigbee, kinda?),

Yes, there are many. You probably know them as "remote control". Your TV, your garage door...

Home Assistant can also control them via gateway devices, turn them into "smart" devices and include them in larger automation scenarios.

I never understood the want or desire for such things honestly

Also you don't want proprietary cloud based products because of things like this:

https://www.newsweek.com/amazon-smart-home-brandon-jackson-echo-racial-slur-allegation-1806947

Wifi smart devices can have that sort of completely local control, not just zigbee. LIFX devices use local control if you don't connect them to the cloud. However, you're then limited in lots of ways, such as you can't then use a smart switch from a different manufacturer to control the lights. Home Assistant takes over the job of Google/Apple Home, which allows different manufacturer's devices to all work together harmoniously. Those services also provide things like automations, turning the lights on when your smartphone arrives within a geofence for example. HA can do even better because those automations will work across Android as well as iOS. It also maintains the advantage of just one app to control your entire home.

As well, as far hardware, I think you're misunderstanding a bit. Nabu Casa, the org that controls the open-source HA project, sells a couple of pre-built devices that run home assistant already. They're designed as turn-key solutions for people with less technical know-how, and provide a bunch of expansibility so people don't waste money needing to upgrade. The proceeds from those go back into supporting the projects costs. But you can go out and buy a Raspberry Pi Zero 2 W (512mb RAM) for $15 and it'll happily run the kind of basic setup you're after. But you will almost immediately run into it's limitations if you try to do anything more complicated.

My HA server is running on an x86 VM with 2c/4t and 8gb of RAM to itself. Have a full music server running on it serving ~6 devices around the house though. Edit: 6 fixed devices. It can also be cast to a bunch of places from mobile devices. My music collection is in FLAC so it's transcoding to lossy on the fly where needed.

I have a proxmox hassio VM with 2gb ram and it runs everything smart in my house. The main purpose would be for automations IMO. Like when my phone chargers at night the house lights, TVs locks all shut down and the cameras go into alerting mode. Home assistant is amazing since you don't have to have 5 apps to control stuff and your data is completely private unlike when using Google home etc. When buying IOT devices I would say stick with zigbee, zwave and only buy WiFi stuff if it works locally without having to have internet wccess.