For the maggots who are illiterate, Makima was stated in the previous chapter to be the "Control Devil". The Reason she is so blatantly OP is because her ability isn't straightforward like Denji's or the fox devil. The "Control Devil", or at this point Makima, is so strong because people fear the idea of being controlled, being puppets, being simply just cogs inside a machine. You, me, everybody fears not having control over their actions, and you hate being told by your parents to "do this thing" or "take out the trash". Nobody likes being controlled. Some people go as far as to FEAR being controlled. That is why the control devil is so strong. As for her ability, it seems like she can control whatever person she enters a contract with, and whatever contract that person has is now hers(without the price). It also seems the reason she cant die is because people fear control to such an extent that the fear of being controlled is so strong that Makima is basically immortal(maybe).
For the contracts that she can use, the reason all of them are called "antemortem" contracts is because it seems like she needs the people she enters a contract with to die to use their powers, thus resulting in you seeing: Yutaro and Michiko's Punishment Devil, The Spider Devil, the Angel Devilman, the snake devil's girl, and Aki Hayakawa.