Goby
ventral fins.
it is modified into fins
Probably, dorsal, ventral and tail fins.
The pectoral fins in both sharks and body fish are Lateral, which refers to them being located on the sides. Ventral would mean that the fins are on the bottom of the fish, which is not true.
there a part of a fish that makes them swim
The opposite term is 'ventral' - For example in fish (and dolphins given the category) the dorsal fins are on the top of the body, while the ventral fins are on the underside.
There are two sets of "paired" fins. The ones on each side of the fish behind the gills are the "Pectoral Fins" and the ones in front of the fishes vent on the underside of the fish are the "Ventral Fins". Sometimes these fins are wrongly refered to as "Pelvic Fins".
Rudd
The "ventrals" is simply another name for the fishes "ventral fins" These are the paired fins in front of the Vent (aka Anus).
They are on the Ventral side, but you probably mean the Pelvic fins and possibly anal fin, depending on the fish species These links provided below may help you :)
It is the skin on the ventral side of the fish behind the mouth, from the gill juncture to the pectoral fins. it is where you would imagine the throat to be.
Female bettas have smaller ventral fins and an oviposter. Males have longer fins (expect plakats) and are usually a little larger.