I am an armwrestler. I just started about 2 years ago. I am very fortunate to train with the top guys in the world--Team Relentless. This group of people includes Dave Chafee (currently the #1 ranked armwrestler in the world), Travis Bagent (2016 overall WAL champ), Bart Wood (WAL's top Referee), James Wagner (2nd place in the world in his weight class in 2010), and a bunch of other pros, various state champs, etc.
When it comes to training, you need to prioritize arm wrestling training over bodybuilding and powerliting training, if you want to maximize your progress. The most important strengths to develop are your back-pressure (which is essentially curling strength, particularly with a hammer grip), cupping strength (wrist curling), pronation strength (twisting the wrist inward), supination strength (twisting the wrist outward), rising strength (pulling the wrist upward in a hammer position), and finger strength.
Secondary strength development would include back strength (pulling the elbows in toward your body). This is secondary to biceps strength because the biceps are always the weak link when compared to the back. You need core strength--particularly the obliques, as most of the pressure in an armwrestling match comes from the side--a type of twisting resistance. You need a particular type of chest strength in order to keep the elbow tight aginst the body, so it does not get pulled out to the side. This means your chest needs to be strong in the fully contracted position--with the elbowas pulled all the way in. Imagine completing a rep on the peck deck machine in the fully contracted position, with your elbows directly in front of your body. This is the position you need to be strong in in order to hold your elbow stable during a match, so your opponent doesn't pull your elbow away from your body.