Paint System is made up of Channels which each contain their own Layers.
In the Shader Editor, there is a Paint System Panel that allows users to open the layer Node Groups
Color Channels are the most common channel you will use. Some templates come with more channels normal, metallic, roughness. If your setup does not have said channels, you can add them and choose a channel template.
You can click this button to add a layer

For more information about Layer Operations (Copy, Paste, Paste as Linked, etc.)
See Layer Operations

This layer allows you to paint on top of your 3D objects.
Each Image layer can have a different UV map, but will still work as expected despite this.

When adding a new Image layer, you can choose the following options:
New Image Layer Let Paint System create a blank imageImport Image Layer Load an image from your computerUse Existing Image Layer Use an image that already exists in current Blender file