How can I be sure that a multi-bit-per-symbol encoding schema exists?
There are three elements of a carrier that you can modulate: the amplitude, the phase and the frequency.

A very popular digital modulation scheme uses one of four possible phases (QPSK). So it can convey two bits on each symbol.

Other often used digital modulation schemes use combinations of several amplitude and phases. For example, 16QAM can send one between 16 possible combinations of phase and amplitude. So each 16QAM symbol can convey four bits.

There are other digital modulation variations similar to those mentioned, like 8PSK or 64QAM, 256QAM, etc.

To be able to decode a multi-bit symbol you need rather complex receivers. So those multi-bit per symbol protocols need mechanisms for data synchronization, you have to analyze the path to know if the SNR is enough to differentiate each symbol, etc.

This is it really in a nutshell, I hope that it is clear as an introduction.