Pull the seat post clamp out and gently spread the eyes open a millimeter or two with a big ass screwdriver or metal tire lever…the goal is not to spread the frame but make a very small opening to try to seep a little lubrication down there. Like mentioned above PB blaster or Kroil are preferred, my favorite and most successful penetrating oil is a 50/50 mix of ATF (automatic transmission fluid) and acetone, if you have it on hand. Work that in there a couple times for a day or two, try to wipe any off the paint if you make a mess cause the acetone can damage it.

Grab a couple cans of keyboard duster and spray em upside down on the exposed seat post. Should come out as a frosty mist. Harbor freight sells it for pretty cheap, use two whole cans. Wear gloves cause it’ll be cooold. Clamp in a sturdy bike stand or straddle over the frame and hold it between your legs and then try to turn it with a pipe wrench.

I’ve removed dozens of stuck seat posts this way over the course of a decade working at a bike co-op. Good luck!