About textures in TGA or PNG form

Hello, everyone! I’m just a beginner on textures. Recently I bought a pack on Asset Store and I wanna use some resources in it. Sadly I discovered that most of the textures are in TGA or PNG form. At first I thought they must have layers so I opened them in Photoshop but there was only 1 layer for each texture.
Here’s my questions. 1.How to extract resources from those TGA/PNG files? Eg. split them to detailed parts.
2.What is the method used for “deliver” different textures to different objects in Unity? Is there sth related to scripts? Cuz I don’t understand how several objects share the same texture file.
Sorry for my “rookie” questions and thanks for your patience.

  1. TGA and PNG are not layered formats, not sure what else you could mean by ‘split them to detailed parts’.

  2. Commonly the textures are assigned to Materials, Materials can be shared