The basic problem is a 'bolt' was originally a cylindrical object that secures something by fitting into a hole.
Fixings with a thread on the end and a head at the other with a shank that fits a hole become known as bolts.
A screw thread, is, well, a thread. By extension, the term 'screw' applies to any threaded fixing.
So all screws are screws. Most screws can serve as bolts but usage is a poor definition as something can be classified differently depending on how it's used.
Only self-cutting screws are definitely not bolts, and even then it is possible to get 'wood bolts' with a self-cutting thread and a plain shank for fixing metal to wood.
One thing that is interesting is that it is fairly easy to make machine screws and bolts. Making self tapping or taper screw threads is much more tricky.