Someone explain how to make a circle in minecraft?

search for a pixel circle generator, type in the diameter and it shows you a block-by-block grid. I just keep it open on my phone while building. for big ones, mark the four points at the radius on each axis first and fill between them, so you don't slowly drift.

honestly the easiest way is to map one quarter and mirror it. place a center block, pick your radius, and build only the top-right quarter going outwards in steps (long runs near the top, single blocks near the diagonal). then copy that shape into the other three quarters. odd diameter gives you a single center block, even diameter gives a 2x2 center. odd is easier to get symmetrical.