Thermostat wiring identification - missing yellow wire/extra wires

Help please and apologies in advance for lack of technical knowledge! I’m looking to replace my old Honeywell 8022C 24v thermostat with a Nest, and have failed on several levels now I’ve taken a look at the existing wiring. The internet doesn’t seem to know the answer so hoping someone has a spark of genius! Furnace is a Rheem rbhp-17j06sh1.

Wired to Honeywell:

There is a 4 wire multiwire – red, green and white are all connected to the expected RC, G & W terminals, with a jumper between RC and RH.

The multiwire has a blue wire which is currently attached via an acorn nut to a separate thick copper red wire (so not wired to thermostat – and it can’t be seen in the photo).

The multiwire doesn’t have a yellow wire; there is another separate thick copper wire which has been wired to the Y terminal on the existing terminal.

So, my questions are: Is the blue wire the C wire? If I wire it to the C terminal (which I would do if I install a Nest), what do I do with the copper red wire it’s currently attached to? I don’t know where that wire goes, so if I need to find out, suggestions on how I do that very helpful!

The other red wire currently wired to the Y terminal is too thick to fit the Nest Y terminal…are there any solutions for this?

Thank you in advance!