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 on their servers, wherever these may be located.
If I’m not mistaken, the standard QR-scanners that are part of th IPhone or Android cameras just execute the scanned QR-godes. The Secuso scanner just scans and displays the code, and asks what to do with it. You have to examine it before use.