I’m a 30 year airline captain and I’ve had one encounter with severe turbulence when I was a young pilot for a commuter airline in a small turboprop. It was a short encounter along the front range of Colorado in the vicinity of standing lenticular clouds. It scared me and the passengers but nobody was hurt and the airplane was undamaged.
Severe and extreme turbulence can be very dangerous to unbelted passengers and crew. Unbelted people can hit the ceiling and just as quickly be thrown to the floor. Things that are not secured can become projectiles and injure and even kill passengers and crew. Best to follow the advice of the flight attendants and keep your things under the seat in front of you or securely in the overhead bins. Always, always, always wear your seatbelt.
An airplane flying into the heart of a thunderstorm could be damaged in rare cases. The turbulence may be so extreme that the aircraft can exceed the design load limits. That means the airplane may be bent or broken in critical parts. In the most extreme cases an airplane might become a total loss even though it does not crash.
In extremely rare cases, the airplane could break up in flight. In 1981, a Fokker F-28, crashed when the pilot entered an area where tornados were reported in the Netherlands and all 17 aboard were killed. In 1990, a Twin Otter turboprop airplane crashed shortly after takeoff in very strong winds in Norway killing all 5 on board. I can’t remember any other accidents that caused an airliner to break up. There may be others.
I’m not trying to scare anyone, the chances are tiny. Most modern airplanes have radar and other technology that alerts the pilot that there is a thunderstorm or extreme turbulence ahead in time for it to be completely and safely avoided. In my early years, there were several microburst accidents but in those cases it was impact with the ground that was the killer, not turbulence. Today’s airliners have doppler radars that alert the crew in no uncertain terms that there is “windshear ahead, WINDSHEAR AHEAD!”
As a passenger, you have very little to worry about in the event of turbulence. It is very rarely any worse than moderate. It’s an uncomfortable annoyance and it feels kind of scary but the crew isn’t worried about it.