That's irrelevant when you're never going to read the source code. It's like saying that you can trust a chinese chemical plant to make good chicken because if you pay for a flight over there they'll let you read their recipe. Unless somebody actually puts that effort in to check that out and conduct a huge factory-wide audit in their own time for free then you are getting zero benefit from that.
One of the reasons I trust F-Droid more than Apple's App Store and Google Play is that F-Droid prohibits proprietary ad and tracking libraries. F-Droid reviewers are also more technically proficient than Apple's App Store reviewers and less likely to err in judgment.
a) open source b) built by fdroid's CI from source without depending on external executable binaries.