Usually something hyper specific. This was a few years ago but I found a very bustling community forum for appliance repair. I posted a question on how to fix my oven and got very detailed answers and technical info involving the circuit board and heating element and troubleshooting steps. Unfortunately the general consensus on there is that for a lot of appliances, the board needs replaced which may or may not be available, and if it is, costs damn near what a new appliance does. Which is obviously done on purpose to drive sales.

The other one I know is my friend will participate on one for modding Toyota Yaris cars.

Bodybuilding / fitness forums are still pretty active.

All of these tend to have subreddit or Lemmy equivalents however.