There are currently 3 popular racing categories
Routes that can go from 50km to 100km. Requires a lot of endurance as riding anything over 50km of technical terrain is very hard.
Consists on a few laps of a mix of gravel and technical terrain where the last 2 racers to finish are eliminated.
Laps of around 4 to 8km. They are high intensity races and the most popular under the UCI.
Over the last few years, most XC mountain bikes are making the shift to 27.5″ and 29″ from the classic 26″. The reason is mostly because bigger wheel size offer better rolling. For some people, 29″ is still too big so they often choose 27.5″.
Most XC riders wear lycra and gear very similar to road riders. The combination of bib shorts and jersey allows them to stay cool and lightweight.
They also wear MTB specific clip-less shoes.