Electronic – Depletion Mode MOSFET Modelling in SPICE

There is definitely a preference for switching with N-channel MOSFETs. There are two primary practical reasons:

1. Electron mobility in silicon is ~2-3x higher than hole mobility. This means that, for a given \$BV_{DSS}\$ and \$R_{DS(ON)}\$, an N-mos will be half to a third of the size of a P-mos, and therefore half to a third of the price (raw silicon cost; this doesn't account for package cost, et al).
2. N-channel MOSFETs switching high-voltage loads can have their gates driven by a signal that is close to ground, obviating the need for a level shifter. (After all, most control circuits are ground-referenced.) P-mos, on the other hand, require a high-side referenced control signal.

As for discrete depletion devices, those are fairly hard to come by. You'll have much better luck looking for JFETs.

How do enhancement mode mosfets even work: You are missing the creation of the "Inversion" layer. When you apply positive gate bias to an enhancement mode mosfet, the existing P channel near the gate contact starts to recede and you have your gate contact - depletion region - p substrate. If you keep going, the concentration of holes near the contact is so little, that you form, in affect a N region near the contact - A N channel now forms between the source and drain contacts.

So at this point, is you examine a cross section of the Enhancement mode mosfet, from source to drain you will see: N+ - N - N+. If you now look "down" you will see Gate - N - depletion - P.