The numbers we use is base 10. Why? because we have 10 digits. 0 to 9.

If we look, 12 is : 2*10^0 + 1*10*1 where 10 is the base.

In base 2 for exemple (mostly use in computer science in pair with base 16) there is only 2 digits : 0 and 1

so the number 5 will be 101 in 2 digits base. why? 1*2^0 + 0*2^1 + 1*2^2 = 5

Now, in bases over 10 the necessity of making more symbols are obvious.

So, in base 100 there will be 100 diferent digits. In this case the youtube video that you post explains the process of memorizing the symbols and i founded it realy interesting.

Hope i explained you in a way you understand

Ohh, just a curiosity: Why we have base 10 for default? Because in the past it made it easy to count by the fingers of the hand