ascii is just a convention to represent letters numbers and symbols
even numbers are represented by numbers
decimal i.e. base 10 numbers (of course equivalent binary used by comp) are used to represent every characters.
it is to note that ascii runs on 8 bits(edited)
some notable numbers
0 null
65 A
97 a
extended ascii charset runs up to 200+ and is as the name suggest some more symbols added
unicode uses 16 bits
16 bits means sixteen 1 0 combinations possible
that’s if you know maths 2^16 possibilities

extended ascii has 256 symbols