What does it feel like to be under general anaesthesia?

No recollection . Patients do not know know, because they are asleep. Some remember waking up. Patients who claim that they recall being under general anesthesia are very rare. It is usually a deep sleep, remembering dreams is possible, but also unlikely.
Very Few Know. With modern anesthetics we try and shut down the memory of the patient before they go to the or. Once they can't remember what happened, it is very hard to find out what they felt as they went to sleep. Without sedatives the very medication that is used to put people to sleep blocks memory formation, so we have the same problem. Same with waking up. Patients come to in the pacu after the op.
No feeling. You are asleep and unaware of your surroundings.