TiggyTheTerrible
sorry i think you misunderstood
Open source apps can normally be downloaded directly as apk file from either their github/gitlab etc. repository or from their website.
Some other apps, like whatsapp, can also be downloaded directly from the whatsapp website.
These app version normally use the developers signing key (relevant for app verifier) and were build by the developers.
You give obtainium basically a link to a source (usually a github repository) and tell it how the release version is called. Obtainium then downloads it, installs it and checks/ can be used to check, for updates.
(Which basically means it looks at the provided source link if a new release version exists)
sometimes you can download fdroid versions of apps on the website of the developers, i am not sure but i belive these use the fdroid signing keys.
Obtainium can not be used to download apps that are only avaible in play store (which is the case for most apps that are not open source) (and i would not recommend to like use something like apkmirror)
Also, while obtainium has a search function, one should first look for the repository yourself (since the names can be different than one exspects)
So it is just a downloader and update checker, using obtainium basically means downloading and installing apks yourself, and includes whatever this usually implies.