In a Swiss001 video, he said that the Twin Otter was a general aviation (GA) plane. But then in another video, he said that the Twin Otter was an airliner. Obviously, he's not a credible source, but it does make me wonder: Can airliners be GA aircraft planes at the same time?
GA refers to why you fly your plane, not what kind of plane it is. For example, John Travolta has (had?) his own personal Boeing 707, which is the first generation of jet airliner from Boeing. When he flew it, it was not under scheduled service, so it would be classified as a "general aviation" flight.
General aviation (GA) is one of several classes of aviation activity, military and commercial being the two other main ones.
The airliner is a class of aircraft designed to carry a significant payload of passengers and perhaps also freight.
Most airliners are used for commercial operation, both scheduled and charter, but not all are.
I might be a little late here but I don't remember calling the Twin Otter a GA plane. I think I have always considered the Twin Otter an airliner but the word GA plane might have slipped out there. Anyway, for that classification, it doesn't really matter what plane you use but rather what you do with it. The Twin Otter is mostly used as an airliner for example to bring paying passengers to remote islands. If you, a commercial pilot, make money with your flight, and it's scheduled, then it is most likely considered an airliner flight. If you, a ppl pilot, rent a plane and take your family flying, then that's a GA flight - even in a 737.