The textures can be .jpg or .png or .dds or whatever. Everyone uses DDS because it makes pretty much any picture very small in file size.
You need the png/jpg/dds file types and "materials.json" file to add textures. Welcome to the forums! And don't forgot to read the rules!
He seems to have taken down all his videos - I think YouTube was being annoying at one point, but I'm not sure why it's all taken down. Sorry about that.