Depends on what the arm can / can’t do. I worked up from 0.5kg dumbbell weights (yep, my arm was that weak) to 5 kg (current… just a bit each day). Then there’s a series of twist bars I worked with (search amazon for PhysioRoom twist bars) or equivalent… there are 4 strengths… then a series Rubber rings for forearm (by squeezing hand). Then work on your SITS muscles around the scapula ( Supraspinatus, Infraspinatus,Teres minor, and Subscapularis) easy to look up… this is a much harder area (often neglected but essential) to improve. Pushing the arms of a chair down trying to lift yourself off a chair is one way, then you can also pull the arm of the chair towards your back… this video should help
Then there are a series of isometric exercises which I posted here last year… have a search with the magnifying glass. Then CORE training… like oblique muscles using resistance bands, also good for triceps. If you need to ask anything please do, and enjoy working with your body !! Ps Brain.rehab is a great site if you’ve had a serious stroke especially, and I love Qigong for rehab, strength, fitness, meditation and health all rolled into one!!
Good luck, Roland