Threaded inserts are great for melting into FDM prints ... Off course, you can also print the threads directly, screw a bolt into an un-threaded hole, or tap a hole. Following his usual rigorous ...
You can use a screw to bite into plastic. You can create a clearance hole to accommodate a bolt and a nut or even build in a nut trap. You can also heat-set threaded inserts. Which is the best?