F-Droid is a free, open-source app manager for Android phones. It allows you to install countless free and open-source applications, including Session, Cheogram, and Amethyst.
The programs in F-Droid's repo are free, and all of the apps (unless specifically noted) can be audited by anyone.
Download the F-droid.apk file from https://f-droid.org/F-Droid.apk Then, open it, and install the app. Most likely your phone will prompt you to allow apps from unknown sources. There is a very high probability this is safe: it's one of the most popular sources of FOSS available to Android users and has a very large user base.
F-Droid will not only automatically select the proper apk version for your device, it will also manage updates automatically. In addition, the app allows easy access to the F-Droid's entire repo of free apps and tools.