To give an example, there is a lot of free open source software mirrored all over the internet, mostly on ftp servers, but also on http, rsync, etc.

If you use Linux or BSD you probably are using some of this software. If you use the www, then you are probably accessing computers that use this software. If you drive a new Mercedes you are probably using some of this software. There are a lot of copies of this code in a lot of places.

Is that centralized? Does anyone hosting a mirror ("repository") "own" the software? Is it the same person or entity hosting every mirror?

Compare Google's copies of everyone else's data, also replicated in a lot of places around the world. Who "owns" this data?