N-bit machines can use N-bit lengths for strings in memory. If you're storing strings your storage format has to specify a way to decide the prefix length (or you can use some other system).

For example `std::string` uses `std::size_t` as a length which is 32-bit on 32-bit machines and 64-bit on 64-bit machines. When you serialise a string to protobuf (for example) it uses a variable-length prefix. Other formats could have fixed 4-byte prefixes or whatever they want (even null-terminated).