Difference between Github, Gitlab, Forgejo ?

Another "Differences in Linux" question :)

I often wonder, what exactly is the difference between this services?

I understand, that:

• github.com is a company, where as gitlab and forgejo are (softwares)?
• They all "manage/wrap/interface with" git?

Questions:

• what software does github.com use?
• whats the difference between them (pros/cons)?
• what about self-hosting? Possibilities/Preferences?

As always, thanks beforehand :)