When vs. By the time

When and By the time are both two time adverbs which there is a slight difference between them.
The former implies " simultaneity,at the moment " but the latter would mean that one event has been completed before another .

- She was cooking the food when we returned house.
- She had cooked the food by the time we returned the house.

In the second sentence, the cooking had been done at some point before she returned.
Notice that by the time can not be replaced with when or vice versa .