My grammar book has a picture for prepositions, and one of them is the one where a guy is trying to open a door
right in front of it, and the preposition is 'at', but doesn't 'by' also work? Before I thought 'by' the door is when you stand at the edge part of it. What's the difference between when you stand in front of it and at the edge side?

ex)Tom is standing _at(by)_ the door.