F-Droid
A store for open-source apps. The recipe is public. Official site only: f-droid.org. After it is installed, search Termux and Termux:API there, same store for both.
Three doors
Software arrives in three ways.
An official store. An open-source store. A file you downloaded yourself. This course uses F-Droid for Termux on purpose. Play Store is not evil. That one copy of Termux is simply the wrong copy.
What open source means
The recipe is public. Anyone may inspect how the app is made. F-Droid specializes in that. Public is not the same as “safe no matter what.” It means you are not asked to trust a sealed box with no window.
Unknown sources
Android asks before a browser or file app may install an APK. Allow that one source for F-Droid. You are not turning the phone into an open door. You are unlocking one drawer so the right store can sit down.
Try this
Say it back: “I will get Termux from F-Droid because the Play Store copy is the wrong copy.” If that sentence feels clear, go to permissions.
Next step
Android locks drawers until you say yes.