I found thermal socks at an outdoors store! They're thin like a regular boot sock, so not at all bulky but they have these silvery threads. I find they keep my feet nice and cozy in the really cold days! After my ride, I usually put in the hot packs in my boots though :)
I either ride in super thick socks (that aren't to tight cause they'll make circulation difficult), or thinner socks with toe warmers so the heat comes through the sock.
No stirrup work also helps cause your feet aren't sitting on metal.
I also get off very slowly cause it hurts so much when your feet hit the ground cold lol.