If the courtesy light is on, the battery will gradually drain. Same as leaving any other electrical item on. If you run the battery too flat it wont have enough power to start the vehicle. Then you need to jumpstart if from a good battery, or recharge with a mains powered charger.

The alternator doesn't drain, it makes electrcal power when the engine is running, to rechange the battery.

So yes leaving the door open CAN drain the battery, if opening it turns on any lights.

There's nothing to drain an alternator. If the engine is running, the alternator is producing electricity. If the engine is not running, then the battery supplies the electricity. If your doors are open and the interior light(s) is on, you are draining your battery.

Battery.

If it makes the interior light stay on, then yeah.