With your personal vehicle access device, aka, the car key. Immobilizers with transponders in the key have been a thing (and in some places a legal requirement) for like three decades.
They've just gotten more aggressive now with "keyless" entry and being able to use your phone as your key, so some validate that info in real time - no network, no access. (Up to a point. They won't immediately strand you just because you ran out of cell coverage obviously, but apparently Porsche did enforce some part of their system to that point)