IPadOS to iOS

You don’t.

iOS for iPhones.

iPadOS for iPads.

The two have differences, now.

iPadOS has features that more suit the larger screens of iPads.

iOS has its own specific features for iPhones.

This is the way it has been since iPadOS 13.

You can't. For versions 13 and later only iPadOS can be installed on an iPad. Before iPadOS 13, it was iOS.