Honestly - depending on the hardware and the specific game - sometimes better than on Windows.

For Steam itself, the experience is functionally identical up to a point. There's a "start" menu entry, it checks for updates and it has a tray icon that allows navigating the standard library/store/chat dialogs. In order to run games that aren't native to Linux, you need to essentially check a box in the master settings to allow the Proton compatibility later that can (usually) run those games on Linux.

What it's running on and the specific game matters though, since some hardware isn't always compatible or requires additional work to get running, and some games won't work on Linux. Usually the latter is due to Windows-specific anti-cheat or DRM (ironically,often the same DRM also f***s up Windows systems or causes performance issues). Regarding hardware: AMD graphics cards are the easiest to get up and running with. Nvidia is getting better with newer devices. Intel is generally pretty good for compatibility though I've had to mess around for some of the Arc based cards in some cases.

It's usually peripherals that can be a pain and especially with newer stuff. For example, there is no built-in kernel module for the Ethernet card on my current motherboard. Getting it up required finding source for the driver online, and running some commands to compile it for my kernel. Annoyingly, the vendor's code doesn't work with DKMS (which would do this automatically on kernel updates) so I need to re-run this whenever a newer kernel comes out. Eventually the driver will likely make it's way to the mainstream kernel and then not require manual work. My previous several boards never required such manual intervention, but this one came out recently and I hadn't checked in the specific hardware.

One other issue is games on Steam that require a 3rd-party launcher (EA, Ubisoft, etc). These sometimes break when updated and often aren't so friendly with offline play without an Internet connection. That said I've played through plenty of EA/Origin games and an Ubisoft (at home with wifi) just fine.

Most games say whether they're Deck compatible. If they are, that should also mean they're Linux compatible. You can also check https://www.protondb.com/ for compatibility ratings.

While this all sounds like there's a lot of BUT's, I've been using Linux as a desktop for years and since the Deck came out compatibility has gone up FAST. I used to dual-boot Windows for a few games - mostly VR - but now even those games tend to run fine on Linux. It's mostly the 3rd-party stuff or shooters with heavy anti-cheat that area an issue. You won't be playing the newest Battlefield or Fortnite, but there are plenty of AAA games and others that run just fine on Linux (and again, sometimes better than Windows at least on my devices)

Many games just work. The ones that don't, don't bother and your g2g.

If you really want the games that don't work just dual boot or playstation.

If by "port over stuff" you mean play your games, then you generally are going to do the same thing you would on Windows - go to steam, click the game, and click install. 98% of my library that I have tried playing since switching to Linux worked just that easily. A couple of games that I bought in early access or as soon as they released, I went into the properties for the game (in steam) and changed the compatibility setting to use "Proton Experimental" instead of whatever the default version was and then they worked fine. Disclaimer I do not play games that require kernal anti-cheat and have not for several years on principle. Many of those do not work in Linux as an active decision to block by the developers.

Now, if by "port over stuff" you are meaning you want to move your save games and the like, they might be a little trickier as the file paths will be different on Linux than Windows (because you don't have the same OS file structure). You can usually find where they need to go relatively easily, and if you have been using cloud saves in Steam those should come over just fine without doing anything.

The Steam Deck is Linux.

Part of the huge movement towards Linux is that Valve have been pushing HARD to make Devs make sure their games work on Linux

Proton has been a game changer, I've been using opensuse tw for about a year and a half now and the number of issues I've run into running steam games I can count on one hand.

Mostly it just works. The Steam Deck runs Linux and they've put a lot of effort into making as many games as possible work on it. At the same time, AI has made video cards work very well on Linux too.

If you want the latest mmo shooter it might not work but so far every game I've played has worked with no hassle.