If you can figure it out. Lemmy at this point is probably still straightforward, but for example go try to compile Android. Just compile it. Last time I tried was 2018, but I spent two full days trying before I gave up. And Android is open source.

There are ways to obscure and gate the codebase even if it is open source.

And you could fork the code, if the original project goes in a direction that isn't popular. Q.v. LibreOffice and OpenOffice, NextCloud and OwnCloud.

The power of open source/copyleft is that it can't be owned as such.

There are a couple of caveats, if there aren't enough developers on the forked project, it will wither. Also, there tends to be lots of fragmentation, as different visions take things in different directions (not a singular project, but what's your favourite Linux distribution?).

Having said all that, each instance is running on someone's hardware, and whoever is paying the bills has a lot of sway for that instance. As you say, since it's open source, there is nothing really stopping you as an individual being that person. A small instance with a user count of 1 is going to be fairly cheap to run. Personally it's another thing to keep up to date. Maybe with a Docky loader…

you can also read it duh