273 questions...a few of them

The feeler gauges are simply a method of measuring piston/ valve clearance if you don't have other measuring instruments. With weak springs installed, you can turn the crank slow, or a little at a time, and push your finger down on the valve and see how much you have before it hits the piston. By stacking increasing amounts of feeler between the valve and the rocker, you can find out what this clearance is. You'll have to "play" because minimum valve/ piston clearance does NOT happen at TDC

Obviously, IF the valve IS hitting the piston, and IS what is stopping the works turning, there will be NO clearance, but you can see/ feel that too, by pushing down the valve and finding that it won't move!!

Try not to overthink this. This is simple mechanical "stuff" and by turning slow, checking as you go, you will eventually find it.