You don't grok it. Bytes are not ASCII strings. They are NUMBERS between 0 and 255. Strings are strings. DECODING bytes with ASCII encoding into TEXT is not "casting", it's DECODING.
If you make it not print strings as "ascii" but as hex, people grasp the difference easier.
Better say "bytes are not Unicode strings".
>>> b'asd'[0]
97
>>> b'asd'[1]
115
>>> b'asd'[2]
100
>>> text = b'asd'.decode()
>>> text[0]
'a'
>>> text[1]
's'
>>> text[2]
'd'