first uart is not a protocol, it's a actual chip in the device.
I'm not a expert, but you can't send limitless data packets because something somewhere would run out of memory.
Basically there is a start and end packet, then your data.
These need to be small, uarts are slow compared to current technology, also there is a size limit. It's like 8-9 bits. you can then send another packet to infinity.
you also need to be able to check parity and resend those packets if any are corrupt.