Here is a more detailed explanation: https://privsec.dev/posts/android/f-droid-security-issues/

Accrescent is a new appstore that fixes all these issues but its still in alpha stage and has 11 apps right now.

I replaced fdroid with Obtainium that pulls apks from github,gitlab,fdroid,etc and it has support for auto updates. It's a little better than Fdroid but still has its own issues.