I've recently acquired a 1998 Buick Regal ( 3800 v6 L36 ) from a friend who had said that he overheated and upon trying to start the car again it wouldn't crank. I have read that there is a problem on these buicks with coolant getting into the cylinders by way of the intake. When I try to start it, I hear a sort of grinding noise, as if the teeth of the starter is skipping over the teeth of the flexplate. I removed the starter, had it tested (passed and teeth look fine) then I inspected the flexplate which also looked fine. Next I removed my accessory belt and tried to turn the motor by hand. The motor doesn't give me much resistance until about 1/2 of the rotation (compression and exhaust stroke?) Could that be what they call a seized engine or bent rod??
A seized engine is when the oil has become thin due to excessive heat or lack of oil resulting in metal upon metal contact.
This can cause items such as the piston rings, bearings, or rods, basically everything the oil touches to experience increased wear to the point the engine "locks up" or seizes.
A compression tool can be had easily enough. It may be rented in some cases also. Once you have the compression tool you can check the compression within each cylinder as someone turns the engine over by hand. If you do not have someone to help you recording the compression gauge via phone will let you turn the engine over then go back and check the video.
The 3800 engine series a very stout engine. They had a plastic plenum(intake) that would crack. A compression check will also show signs of a head gasket leak through this plenum or other sources.
Signs of a head gasket leak are milky oil after the engine has been run, excessive coolant use, or white smoke after the engine has reached operating temperature when the outside temperature is above freezing.
But with you not wanting to run this depending on the amount of coolant in the oil you may be able to drain the oil with the engine cold and watch it going into the drain pan looking for the black or bronze oil to change colors to clear or coolant color.
When you rotate your engine it takes 2 rotations to cycle all 6 cylinders. If you rotate the engine and experience the resistance briefly that could be a single cylinder scoring the cylinder walls. If you experience it multiple times that could be bearings having a "flat" spot. The only way to know for sure is to tear down the engine. Using a compression tool and the external checks above you can get an idea whats wrong before pulling the engine.