How are RS 485 transceivers implemented?
Assuming the receiver side of a transceiver is boring to you (just a comparator with some hysteresis):
TI's DS9638 (datasheet of the SNLS389D) actually has an equivalent circuit for a single transmitter circuit:

To illustrate what's happening a bit better:

In green, the identical output stages. Look at the left:

Q15 has the job of pulling the output low as soon as the base of Q6 sees super-threshold voltage
Q18+Q19+Q17 form a current amplifier
rest of the combination of Q16's collector and Q18 base invert

The whole functionality of the center part is to make sure Q5 and Q6 see the inverse, but at very tightly controlled thresholds.