How should I fix these openings?

[Gs As]

There have been doors that I've now removed, and I just want it to be smooth and nice without installing new frames, moldings, etc.

What material should I use to make it durable? And tips and techniques to make it as good as possible?

 

[Testarn]

I would build a frame of studs that fills the width of the wall minus the board material, probably with a vertical stud in the middle of the hole. Screw it into the wall on both sides with appropriate plugs and screws in masonry or just screws in wood.
Then add OSB+drywall, or just drywall before you plaster the joint between the wall and the patch. To prevent cracking, a strip of fiberglass/microlit or similar can be embedded in the plaster to reinforce it.