Why the exact same transistor acts like a variable resistor in one region and a current source in the other — and why the name "saturation" is the opposite of what BJT experience trains you to expect.
A single MOSFET, with a single Vgs applied, can behave like two completely different components depending on nothing more than how much Vds is across it. Push it into one region and drain current climbs almost linearly with Vds, like a resistor whose value you set with the gate. Push it into the other region and drain current goes essentially flat — it stops caring about Vds at all and instead depends only on Vgs, like a current source. Nothing about the physical device changes between the two. Only the relationship between three voltages — Vgs, Vds, and the threshold voltage Vt — decides which behavior you get.