A disk or cylinder could be a very efficient shape for a spacecraft if aliens from planets with different gravities want to travel together. Flying saucer sightings frequently involve something rotating. If this is real centrifugal power would have a similar effect to gravity but the force would be different at different distances from the centre. Therefore different aliens that evolved on different planets or moons with different gravities could all have homes somewhere in a flying saucer where the simulated gravity fits them. If a flying saucer picked up humans somewhere in the craft would be an area that simulates the gravity of the Earth. What about flying saucers with a hub in the centre? Perhaps that’s imaginary. Perhaps the hub is for some alien machinery that we don’t understand. Areas in the disk where the simulated gravity doesn't suit any of the travellers could be used for storage and sections could be cleared if new aliens are discovered who need that level of simulated gravity.
To simulate gravity a rotating disk would probably need to be very big, say at least a hundred metres in diameter. A smaller, faster rotating disk could also simulate gravity but there would be significantly different gravity in different parts of the same room. Depending on the physiology of the aliens this could cause problems. The above can explain very large flying saucers or flying saucers in the sky where the real size is impossbile to estimate. Smaller flying saucers need a different explanation.
None of this shows that flying saucers are real but it shows that flying saucers are possible.