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.
Generally, GrapheneOS ia pretty much the best privacy-focused android version you can have, as it will strip away anything that’s related to google while focusing on a small amount of supported devices to ensure that not many vulnerabilities are possible on the devices.
LimeageOS are less focused on privacy and more about availability. /e/OS is pretty much a downstream of LineageOS focused on Fairphones. Generally a good choice if you want android without google services.