The fixed vagina is in-built into the body of the doll and contains a texture, much like that of a real woman’s vagina.
A removable vagina is an insert that can be removed for cleaning and washing (sort of like a Fleshlight). The vaginal cavity of the doll with this option, is empty around the vagina area, with the insert being textured on the inside and it slides into the vaginal cavity. The external appearance of the vagina is the same whether you have fixed or removable insert vaginas.
Most would think that the fixed is better for experienced users and the insert is better for first-timers as it is easier to clean. However, there are lots of people complaining about the incorrect orifice direction with a removable vag, it is recommended to go for a fixed vag.