Without meaning to disregard the position of others on fdroid being insecure, i have argued and would argue that is not true.
Downloading from fdroid is as secure as downloading from github.
There was a technical argument laid against fdroid, however i have critiqued this argument already and have not heard any meaningful counter argument against it yet.
Bottom line: fdroid has some potential issues however those issues do not apply in all situations and the categorical statement that "f-droid" is not secure is simply not true. Downloading from github is not safer than downloading from fdroid. In fact I might argue the contrary, or at least parity.
Do know that fdroid is not a replacement for play store as most apps on playstore are not available on fdroid. Also many apps are fdroid are not available in playstore. So they are "different" libraries with some crossover. Also some apps are only available from fdroid.
Don't use the official fdroid app, use droidify instead. Also be mindful that the app you download adheres to recent target API levels.
Understand what you are downloading from fdroid and you're good go.
For a guide to get you started on apps and stores read this:
https://discuss.grapheneos.org/d/5267-basics/9