Install Obtainium from F-Droid to manage app downloads and updates

Go to Obtainium -> Add app

Paste the URL of the official download page: https://store.steampowered.com/mobile. Remember to always check that you're using an official and safe link to a webpage or supported source

In field "Custom APK Link Filter by Regular Expression", write the pattern under which Steam uploads its app. This is important because "/mobile" page has 3 different apps. The pattern is: /steam-android/steam-(.*).apk$

(Optional). To improve update detection, paste "/steam-android/steam-(.*).apk$" (w/o quiotation marks) to "Version String Extraction RegEx" and write "$1" into "Match Group…"

Use Steam