Everything is 0s and 1s to a computer. What a pattern of 0s and 1s encodes is decided by people–often arbitrarily. Over the years there have been attempts to standardize encodings but, for legacy reasons, older encodings are still valid.
The 0s and 1s that encode ’ in UTF-8 (a standardized encoding) are the same 0s and 1s that encode ’ in CP-1252 (a legacy encoding).
The � symbol is shown when the 0s and 1s don’t encode anything of meaning.