Modern networks pass data serially bit by bit over a wire. So on the lowest level networking works on a bit level, not a byte level. There is some logical stuff in upper protocol layers that will actually group things by bytes and such, but when you strip everything down to the smallest detail of what networking does, transmit data over a wire via electricity or lights, or otherwise wirelessly, its bits.

The Mbit number your your ISP sells to you is actually lower than what you actually get, to ensure that you get what you paid them for. If you track your bandwidth usage closely you’ll notice you burst higher than what that number it’s “up to” because there are variables outside of their control as well. So it’s not a hard ceiling, but a rate they promise to be available to you.