Can't they make one number data type that increases in memory based ont the length

Answer 1

C++ does not have a standard numeric type for "arbitrary precision" or unlimited size.
There are some libraries out there which implement these features, but you have to look hard and choose carefully.
I suggest to check these topics:

https://stackoverflow.com/questions/64565151/whats-the-c-equivalent-of-cs-decimal-type

https://stackoverflow.com/questions/12988099/big-numbers-library-in-c

Answer 2

Can you please elaborate your question if you have one!? If you're suggesting implementing another data type in C++ then no one other than the creator of C++ would help.