Texture Paint behaviour in regard to a Texture If I have texture paint working, and add a new Texture then it stops working. Or it behaves weird.
It's a bit tough to explain in textual format for me so I made a video about it:
https://www.youtube.com/watch?v=ltLqniomK6U
To explain in short, when I add a texture here, then texture paint stops working or behaves weird:

Can someone explain why this is?
Yes atomicbezierslinger is correct, you can do this by pressing the
Browse material to be linked button, then from the drop down menu, select your UV image you unwrapped from your object.

Similar to how you assign textures from a texture node in the Shader editor.