iPhone auto brightness

I will assume that you are using iOS version 15 on your iPhone 6s Plus since you have not disclosed this important piece of data.

If you are using iOS 15 on your iPhone 6s Plus, then open Settings ➡️ Accessibility ➡️ Display & Text Size ➡️ Auto-Brightness (All the way in the bottom).

Axel F.

Settings

Display and Brightness

Turn "Automatic" on or off