Recently I discovered those social medias as an alternatives to Facebook, Instagram, TikTok and Discord. What makes them different in terms of privacy versus the classic social medias? Are they really more private?
No, social media is not private.
If you can’t find the source code for a social media platform, I wouldn’t use it.
The platforms you mentioned aren’t any better than Meta and the like—they’re just PR statements with no real value.
Stoat does publish source code Stoat · GitHub
Same for communities.social GitHub - danielBingham/communities · GitHub
Their mission statement also sounds interesting and once the yhave achieved their intended legal structor worth to be checked out.
I would encourage you to look at activityPub Social media platforms as some of them have a longer track record. PG recommends Mastodon for example.