Electret

Mems are super cheap and low profile these days

I imagine it's more that it's a cheap way of achieving the effectiveness, vs. a (hypothetical/possibly extant, no idea) that doesn't use one.

'N95' is just a rating (roughly the same as 'FFP2' in Europe) concerning the rate of filtration.

Yes, it 'works as a protection from the outside', they block those particles they are rated to protect, which (by diameter of particle) includes the coronavirus.

Scarves and pieces of generic cloth or polyester sold as 'coronavirus mask' obviously do also protect from the outside, they're not somehow directional, it's just that they aren't particularly effective at relevant particle sizes, to put it in the extreme: they'll catch your sneezes. Of course they'll catch others' sneezes too, but usually people don't sneeze on your face, so it gets talked about as inside/outside as though direction matters.

I'm well familiar with the Lorentz force! Did homework finding paths charged particles take through fields and everything. What's novel to me (if obvious in retrospect) is that a nontrivial electric charge/potential can be frozen within a chunk of dielectric. This is as fundamental as magnetism, and I'm somewhat buffaloed by the fact that electrets aren't more commonplace novelty items.