There are only 21 million total bitcoins that can be mined, what will happen if all bitcoin is already mined?
There aren't 21 million bitcoins. The number of bitcoins is unlimited.
Why? Because right now there are 2 bitcoins. bitcoin cash and bitcoin core. Bitcoin forked into 2 bitcoins. There are now 42 million bitcoins. And it can split again and again. There is not "official" bitcoin because bitcoin itself is decentralized. Each time it hard forks, it splits into another 2 bitcoins, both of which can be mined. And this can go on forever.
So, there is no limit to the number of bitcoins and there is no "official" bitcoin. It is just whatever bitcoin fork is popular at the time.