Usyk definitely had a lot of fights as an amateur. Like Vasiliy Lomachennko, the best amateurs from that region have a lot, including at the international level. The Ukraine amateurs sprogram is very similar to Cuba's. Keep the best extremely busy, fighting as frequently as possible to stay sharp. To do that, like the Cubans, it takes a great defensive ability, or else they would have taken way too much punishment. (Headgears mainly proct the ears, and prevent cuts .... but punches still hurt like hell. My brothers & I had a friend who died from an amateur fight, when he was put in way over his head, about 50 years ago.)
They fight the best in the world on the big stage, but also get a lot of fights with guys that are on a lower level. There aren't enough top guys to fight 300+ bouts at that level. So they include mid- and lower-level guys, and that is often to develop and practice their defensive skills with, while outpointing or stopping the guy. That still takes a toll on the body -- as does sparring and other preparations for fights.
A huge factor is the trainer, both in the gym and corner, as well as the referee and ringside doctor. It's just my opinion, but too many pro fights are allowed to go on too long. It happens in the amateurs, too, but not as frequently as it did when my brother and I fought. But I like both Usyk and Fury's trainers. I don't expect either to continue for much longer As Marvin Hagler said, it's hard to get out of your bed at 3 am to do roadwork, when you are sleeping in silk sheets. I don't think either has sustained serious head injuries, though they have taken punches that aren't good for any brain to take. But I'll add to Hagler's point that it gets harder when all your joints ache.
I fought over 300 times, while my late brother had about 85 bouts total. With each of us, it included fighting a wide range, from skilled fighters to club fighters. My brother had good defensive skills, until he took the first punch. Then he forgot them. He won the majority of his fights, around 85%, but he took a heck of a lot of punishment. He had a rough time as the result starting in his early 30s, and it eventually put him down for the count. I had a 99% rate of victory, and took relatively few punches. In a lot of fights, opponents didn't land a single clean punch. But these days, I feel it in my joints, and especially my left ring finger, which I broke several times. Too stupid to get it set correctly when I was young, and did it myself. Yikes! But my mind is relatively normal, I think!