A whole lot of teeny tiny semiconductor switches that can be set to on or off. I’m oversimplifying. Google it if you’re curious.

Old school hard drives used magnetic discs to store data. Modern SSDs don’t use magnetism to store data. It’s safe.

M3 Pro. The main difference between Pro and Max (of the same generation ) is more GPU cores on the Max. For programming you don’t really need much GPU power but a faster CPU will speed up your workflow (namely improved build times). The newer M3 Pro should outperform the M1 Max for your needs.