How are blockchain addresses pre-existent?
An address is an encoding of a 20 byte value. This value is just a number, from 0 to 2^160 (there are 8 bits in a byte, so a 20 byte value has 160 bits and a bit can be 2 values). Because we know what all of those numbers are, all possible addresses are known beforehand and already exist; they are just a number.