You should stick to JavaScript for web. Running Python in a GitHub page is technically possible, but I really don't recommend it. Python and JS are very similar languages, right in the same family of single threaded interpreted C-style languages. If you learn one, the other is almost identical, with slightly different syntax

To answer your question, GitHub pages cant run a backend service, no. There's a good chance you don't really need one yet. Your GitHub page can load publicly hosted JavaScript libraries just with a URL, and it can use existing third party services. For example, you could use a free Google Firebase account to enable login/accounts, and never have to build a backend for it yourself

When you do decide to write backends, you can do that in JavaScript too, using Node.js. It's real easy, but there is a lot to know about networking, concurrency, server management, etc. But you can also write Python backends really easily. My favorite language for this is Google's Go

I'm gonna say -1 to the suggestion of replit - never heard of that service and appears to be some AI startup. If you wanna do backend, just head to Google Cloud or Amazon Web Services. Google Cloud is my provider, and they have a lot of great options. For example, you can make a "Cloud Function" which is a type of "serverless" technique - you just write one little blob of code, set up the configuration, and it gets magically executed when conditions are met or a request is made. Then you can skip knowing 99.9% of what goes into making the magic work and just write the code. Or when you want more complete control, you can look at Google Cloud Run for a pretty easy full site/backend system, and Kubernetes for the really advanced modern tech. Google gives out a ton to free computation hours, so you can likely build a full backend without spending anything in cloud run