The best way to install them is through the F-Droid store, which is a catalogue of FOSS software for Android. It’s installable by downloading the .apk file linked on the front page of the F-Droid projec’ts website. The mentioned apps from the Karlsruhe Institute of Technology can then be found by searching for “SECUSO”, which is the name of the research project behind them all.
Alternatively, you can also get them through the Google Play Store under this link or again by searching for “SECUSO”.
In particular, I recommend getting the QR code reader, because many of the free-to-use scanners route everything you scan through their servers, so they’re obviously collecting your data.
There’s Sailfish but I think it still has the same limited handset problems that most of the others have
I bought a used Pixel 8 pro off swappa specifically to run Graphene OS.
Honestly, Sailfish is only worth it if you have a license, as there are very few native apps and the android compatibility layer is the only way to have a proper daily driver phone. I did try it with the Sony Xperia X II a 1-2 years ago, and honestly, it was still very buggy.