I hope I can explain this without pictures.
Assuming a vertical accumulator...
There are two connections, the inlet and the outlet. The inlet connection extend into the inside of the accummulator a couple of inches, then just ends. The outlet connection extends into the accumulator all way to the bottom, then makes a 180deg. bend back towards the top. Looks like a "J." This forces the suction connection to draw vapor from the top of the vessel. To get oil back, the is a small hole right at the bottom. When the vapor liquid, and oil stratify, if deisgned properly the return bend is sitting in the oil.
This is why you have to size them properly. Undersized means you can't eat enough liquid, oversized means you can't return the oil properly.
check out refresearch.com. They have a good cutaway.
Clyde