how can I join faces like this (in the image):
I want to make all the selected painted faces be just one.
where is image?
press f, and confirm the dialog. It won’t actually make one face, since blender can only handle faces with up to four vertices, but it will let you treat it as one. You probably should move vertices around though, so that there are regular faces or it becomes very difficult to work with.
Alt+j, but to be on the safe side, select the specific triangle pairs that you wish to oin with Alt+j one at a time.