Feedback How to painting a Terrain outside Unity? How do I export import layer mask textures?

Painting a Terrain outside Unity

When we are painting a layer, we are painting a specific channel in a texture. in my case "Terrain 1x1.asset".

How do we extract it?
Is it possible to replace these layer mask textures?
I wish to substitute each RGB channel with my own texture done in Photoshop or else.

The idea of painting a terrain inside Unity as the first option is naive or for a beginner. Instead of brushes, I want to use textures.