I've got the book Armwrestling - how to become a champion by Ernie Jeffrey. It's quite old, but still considered one of the best on the subject. There is a lot of technique involved in armwrestling a good armwrestlers can easy beat a stronger guy with no technique. Explosive response at the start, wrist position & wrist control are the goals. Look up Rob Bigwood, he's a really good pro armwrestler (who's also vegan), a amazing athlete in his field.
For armwrestling you'd still do a basic exercise routine, even things like squats, dealifts will all help. You might need to focus on a few grip exercises & push-ups. Looking through the book they also seem to rate strands (chest expander) style exercises as well (maybe the constant tension mimics the constant tension on a pull a bit more than weights on a bar?).
I'm certainly no expert in armwrestling training - I've never worked with an armwrestler & as I've had multiple shoulder injuries I don't indulge myself in the sport (not to say I won't ever, but at the moment the injury potential outweighs any benefits I might get from it). Maybe someone knows more about it than me who's onboard?