Eagle_Owl Not all of them are compiled by F-Droid themself, but we know what you mean.
Developers can offer their apps via own repos at F-Droid!
They are in separate folders.
These devs use F-Droid as platform to deliver their own stuff, not “manipulated” by F-Droid.
Examples: Bitwarden, Session Messenger, SimpleX Chat, Threema Libre, NewPipe.
When 23Sha-ger (or anyone else) talks about Fdroid compiling apps, they're talking about the main Fdroid repo, where most / all(?) apps are compiled by Fdroid to my knowledge.
And to be quite frank, third party repos on Fdroid are pretty pointless unless the app you want is specifically available only through the repo. Most of the time, an app from a third party repo is just the app from Github (e.g. Bitwarden, NewPipe). Essentially a third party repo "adds apps" to Fdroid the same as users would add apps to Obtainium.