The “Maahir’s Gambit” scenario.

But in this case, is the immateriality of ghosts considered as resistance (to physical damage) or as an innate immunity?

The immateriality of ghosts is just a 50% chance (since we're talking about a program, it's a pseudo-probability) to miss with a physical attack, not immunity (you can still hit) and not resistance, because the damage isn't reduced; it's either 0 or 100%.

I used to think that magic always hits, but in reality, magic can also miss. But it seems to happen less often.

Only Magic Fist misses on ghosts because it deals the same physical damage.