Exercise for Arthritic knees and shoulder

The best way to select a set of exercises for yourself, especially with limitations caused by bad joints, is to get referred to a physical therapist who specializes in rehab for people with arthritis (this is different from the PT who mostly deals with young people with a sports or work injury that can be cured.) The PT can evaluate your exact needs and limitations and work with you to design a program. This does not need to be many sessions - just a few to get you started doing each one correctly, finetune for your body, then maybe 3 months later to check progress.
If that isn't possible, the most helpful exercise I know of for knees is "wall squats" (example: https://www.youtube.com/watch?v=3J99J5NxNSQ) These help strengthen the quads, which takes pressure off the knees., keep your feet far enough from the wall so the knee is directly above the ankle as you squat to avoid knee pressure.
Here are a variety of exercises for shoulders https://creakyjoints.org/diet-exercise/exercises-arthritis-shoulder-pain/ Some of them may not be possible on the arm with the torn rotator cuff.
Crunches and other abdominal exercises are still possible with sore shoulders and knees. Also sitting on a properly sized balance ball, doing deep breathing and seated crunches.
As for joint-sparing aerobic exercise to burn calories, water walking, using a stationery bike, or an elliptical exerciser are all possibilities. Even walking, on even ground, using hiking poles.
Do you see anything here that you might be able to do?
One note - if any exercise causes discomfort, ice, ice, ice afterwards.
Let me know what you plan to do.
Sue