Thermostat install with no control board. G/R/W Easily identifiable but Y spliced into other circuit. Not sure where to put C

Title says it. Not a high voltage system as theres a transformer involved. Weirdly, Y is marked as C on the furnace side of the electrical diagram. I feel like the solution is just outside of my grasp, unfortunately.

In picture, large red is R, large green is G, Large white is W, blue/white is Y, and i have no clue what red/black is.

The wires in the background are 120V romex, make sure you don’t splice the thermostat into those.