What you need to remember
Download from f-droid.org. The first open can take a few minutes while the app list arrives. After that, search Termux here, not in Play Store.
F-Droid itself arrives as an APK. That one unknown-apps prompt is expected.
The right store
F-Droid is an app store for open-source apps.
Open source means the recipe is public. We use F-Droid because the Termux copy on Play Store is old and causes problems. Official site only.
This page is the explanation. The install walk is Stage 1 or the first boxes on Start today.
The lab is Stage 1
This page is the library card. The six-step checklist lives on the Path.
Next step
Termux comes from that store, not Play Store.