They honestly don't most of the time they do things with weights it's extremely light and for explosive purposes, the risk of injury is too high for them to be fukking with weights.
Boxers have long lean muscles, being a big bulky weight lifting type will mean you have awful endurance too.
I had a boxing coach a while ago and he told me to specifically stay off weights unless they were light/ explosive
I'm about to lift a light for a couple hundred reps right now
Most boxers workout to their specific styles. If you're an athletic type boxer than you do calisthenics, plyometrics, high intensity cardio, regular cardio, and box if that's they're style.
If you're a distance type fighter (like most Mexicans) you do a lot of roadwork with your boxing.
But look at Foreman, Shannon Briggs, Mike Tyson, Lenox Lewis, or most boxers from the 80's... They were all shredded