Answer 1

float : 7 digits (32 bit)
double : 15-16 digits (64 bit)

as you see double covers a wider range of numbers, but on the downside it uses more memory.

So when you don't care about memory in your program, like simple and small programs, you can use double. but in very large projects, you have to be careful. Only use data types like double and decimal when you need to store very large numbers.
Otherwise, it's better to use float so you can keep the memory usage at minimum.