How do the chemicals in sunblock work, to keep out ultraviolet rays?

energy absorbtion. Quite simply (oversimply) the opposite of an (electric) stove turning red when hot: energy into coil, electron excitation, photon emission.

The waxes and oils absorb energy, the molecules in the oils change shape/conformation to dissipate the energy (UV rays are eletromagnetic energy).

parasol (not paba) is a cell cycle inhibitor which allows the cell to stop multiplying, and check itself for DNA errors. It does not block uv rays. this is an increasingly common ingredient in sunscreens.