The built-in or fixed vagina is exactly how it sounds. The vagina is built-in to the doll just like a real woman. The removable vagina is a sleeve or fleshlight-esque tube that you can insert and remove from the doll. The tube is held in place by the doll’s labia.
Which is better? Most of our customers prefer the built-in vagina as it provided the most realistic experience, however the removable vagina is easier to clean and replace. If you value complete realism you’ll want to get the built-in vagina, if you want something that is easy to pop-out and clean after use you should consider the removable vagina.