I have no particular knowledge about LIN signal levels.
I do know it's designed as the cheapest that can be built, and still be good enough to be used in "automotive" stuff.
It's also supposed to be used as a very localized bus, for example in a car door, where CAN goes to the door, and LIN controls nodes for a door lock, window motor, and button interfaces.
Is LIN built with some discrete transistors, or are there chips for them?
If there are chips, they have datasheets.
I do have some experience with RS485.
Contrary to what many people seem to think, RS485 needs 3 wires. The 3rd wire is for a ground reference.
RS485 works with differential signalling, where one signal line is (at least) a few hundred mV higher or lower than the other. It is very common to have a common mode range for the signals between -7V and +12V and communication still works, even the signals are several volts outside of the power supply rails of the RS485 driver chips. This is very useful in a shared bus. For example, I use RS485 over Ethernet cables, and also send 24V power over those cables,( upto about 1A) 1A is about the maximum ethernet cables can handle without melting, and at the end of a long cable you may loose a few volts due to copper resistance. Half of the voltage drop is in the 24V power line, but the other half is in the GND lead. This means that RS485 nodes near the end of the cable have their singal lines 2V below their "GND level"!
Blindly assuming it will work is not a good idea.
You'll have to analyze the circuit you use, read datasheets, etc.
PC817 seems to be a common opto isolator. They cost around 40ct at the west side of this ball of dirt (10ct each in full reels), and pennies with free shipping from Ali.
Ali is nice for some experiments, but quality... varies...
Just for fun I looked up the price at LCSC.com.
PC817 can not be sold there in one-offs. They do not do that silly western stuff.
Minimum order quantity is 20pcs, but with their price of $0.02879 it still seems reasonable.
I'm planning to make my first order with LCSC "soon", but have not done it yet.