This is a piece of dialogue about a colonoscopy from the series "Outnumbered" s03e03:
— Yes, but how does it get inside your insides?
— Well, they put it up your bottom.
Why not "they put it up through your bottom"?
The phrase "up your bottom" is a standard phrase, perhaps an idiom, in UK English for "inserted anally". The phrase "up your rear" is perhaps more common in US English. Compare "up the spout" for a bullet in the chamber of a gun, ready to fire (although this phrase has other senses as well).
As with many other idioms, the phrase is not entirely logical, and could be phrased otherwise, but commonly is not. That is just how it is said.