Yeah, I think fdroid is more strict about the apps they include and I think they might do some extra tests (hence why fdroid apps have different signature from git repos of the apps).

Please search it yourself, I don’t remember much to help you.

Here’s izzy’s site: apt.izzysoft.de/fdroid/index/info

PS. You might see Izzy in various github app repos talking with developers asking to import their apps to the izzy repo