Not to nitpick, but define “bodybuilders”. In the late-19th/early-20th century, it was physical culture. There was no bodybuilding.
There were some great-looking physical culturists, but for the most part they all basically found a ton of different ways to move pretty heavy weights mostly with barbells and dumbbells, and some kettlebells; often supplemented the heavy lifting with much, much lighter weights used for different exercises; and some did gymnastic-type bodyweight stuff as well.
Bodybuilding was a thing that gained momentum unto itself in the 1930s and '40s, pretty much in line with the availability of a wider variety of equipment that separated itself from strength-focused lifting, as contests got more specifically organized as physique competitions.
In general, the principles from back then were basically the same as we have now. I covered that in the 5 Timeless Lessons article, drawing parallels between what they used to do and what’s being done now.
I also covered Alan Calvert’s 1924 book Super Strength that covered a truly ridiculous amount of training concepts that are still being done today:
“A small sample of the advice Calvert talked about in the 1920s includes the importance of barbell squats and deadlift variations; doing farmer’s walks and kettlebell swings; using heavy, low-rep compound lifts as well as targeted isolation exercises for moderate reps; lifting with thick bars for grip strength and forearm size; starting with fundamental bodyweight training like push-ups (called floor dips at the time) and bodyweight squats (called deep knee bends back in the day); the benefits of a hook grip; and the idea of “looking like you lift.””
Everything in that list, we’re all still focused on now. Ev-er-y-thing.
To be fair, not everything they did was great. Arthur Saxon advocated this as a grip and forearm exercise. (Basically a triceps extension from that position)