Like you say Obtanium is able to grab the compiled apks from many different sources, like GitHub release pages and f-droid 1st and 3rd party repos.
F-droid, on the other hand,bgrab the sources from many repos, compiles and signs them, so that all the apks from f-droid repo are guaranteed to be compiled from the relative source, without the addition of anything (tracker, malware…). The same cannot be said (in the sense of technically guaranteed, not in the sense that they are not honest) for the apks you get from GitHub, which are uploaded from the various manteiners.
Edit: in a sense “f-droid build system and repo” and Obtanium are different and complementary:
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fdroid is a repo that gives transparency from source to binary of the apks and contains a certain subset of Foss apps
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Obtanium is a system that aims at giving the greatest amount of apps, without looking too much at where that come from
What about ghostracer? Is it Foss?