To get to the bottom of something:
Literally: To reach the lowest physical point – The cat’s trying to get to the bottom of
Figuratively: To get to the base or root of a matter to understand why a situation or problem exists – When she saw the broken window the teacher questioned all the students, one by one, to get to the bottom of the matter.
Other meanings for ‘bottom’
At the lowest point – The cat’s getting the clothes from the bottom of the drawer.
Buttocks, haunches, the part of our body that we sit on – The cat’s bottom’s hanging out of the drawer.
To describe something that’s at the lowest point – The bottom drawer.
So, in the video, The cat’s bottom’s hanging out of the bottom drawer as he’s trying to get to the bottom of it.