Everything is fine with the drain, however there was some gunk left on this picture because no one used it for years and I was cleaning it up when I took the picture, here is my floor drain which was closed off with a cap:
I can't see what exactly happens, but first off this black plastic is not removable – it might be stuck with concrete in the floor. Anyways, the whole idea with this drain is that it's not going straight down to the sewer. First there is water that is always stuck in the larger basin, which drains into the smaller hole which ends up to a flat surface with a small circular 360* open, which then it might (I suppose) continue to the normal pipe, I draw a section view what I think happens here:
So what type of drain floor is this and can I connect bathtub drain pipes to it (although it is already narrow, short and I cannot use reducer).