They don't necessarily do them all in one competition! Our coaches have heavy input into what swimmers swim at comps and will possibly put in 2 200's in a day and something else (like a few 50's or a 100). And then next time do an 800 and a few 100's.
Stroke wise they'll train free and a second stroke (no 1) on some sets and other sets will be form stroke.
Our 15yo girls are just starting to specialise now over stroke as well as distance but some are still trying to work out what their best stroke and distance is. Generally they train as distance swimmers if that's their specialist distance from a younger age.
I don't think there's any hard and fast rule about the age this happens. I just know our coaches say if they specialise too soon (some try and only swim their fave strike from 11/12!) they miss out on discovering that another stroke may be better for them as they grow and develop.
That's why we have sets where we have distance, mid distance and sprinters. Because the training is all different.
At ds club he started at they had 1 squad stream and were allowed to specialise earlier and all of the squad did the same sets.
I couldn't tell you if one approach is better than the other or advise as I'm not a coach! All I have is the information about what ds club do and why they say they do it.
Whenever ds says to me "should I......?" About anything swimming related I always reply "ask one of your coaches!" 🤣