Others are posting the well written explanations, so I’ll make the short comparisons.
GitHub is like Reddit is to Lemmy. It’s the main player in source code hosting, proprietary and centralized to the profits and whims of Microsoft. But for that cost, you can easily bet a project you are looking for has a presence there, and it’s easier for a dev to pop from project to project with one account and identity.
The others are like Lemmy, meant for hosting your own GitHub-like website with all the bells and whistles on top of the standard Git codeshare. There’s a lot of feature parity, though some softwares have more than others. But it comes at the cost of obscurity, Codeberg is a big player but any instance you find is isolated, and any devs you entice to help you need to register additional accounts personal to that instance. And the hosting costs are on you, it can all vanish with an unpaid domain/server bill unlike the central giant of GitHub.
And don’t even get me started on “AI”.
As the family technical person, I can say after years of attempting to teach people to understand and solve their own problems, my support calls are down in the past year! Is it because they got smarter? No! They started using ChatGPT, CoPilot, etc and following it blindly. Do they understand the concepts of what they are changing and doing? No, but as long as the original problem is fixed, who cares if a dozen more are created, as long as they(the problems) keep quiet.
I am cursed to be in the middle, couldn’t just be given a well paying technical job like my forebears, but nobody thinks they need my technical skills anymore, so I have talents now viewed as outdated and of limited use.