Modern ignition key fobs (the plastic part) contain a tiny ID chip inside them, the size of a grain of rice. These are similar to the ones injected into cats and dogs for ID purposes. The ID chip needs no battery, because the lock has a loop of wire around it, which interrogates the chip.
If the chip is missing, or doesn't respond, or responds with the wrong code - the ignition system will be inhibited. Some vehicles make it obvious the inhibitor is preventing starting, some do not..
Some vehicles allow cranking, but inhibit actual starting. Some disable both cranking and starting.
The ID chip can be deliberately prevented from responding, by wrapping some kitchen foil around the plastic part of the key.
One botch was to bypass the inhibitor system by glueing the chip next to the ignition switch, so a perfectly ordinary cut key could be used, without need for a chip.