Welcome! I'm so glad you are trying to gain strength and mobility.
You can do arm lifts and bicep curls with small weights. If you don't have the kind of weights a gym would have, you can start with cans of soup or other canned goods for weights and build up slowly from there. Does your bed have an overhead trapeze? If so, you can grab onto it and raise your upper body up as far as possible.
You can work on lifting your legs up as much as you can - the idea is to work against gravity.
If there any possibility that you could have a home visit from a physical therapist who could assess you and make more formal suggestions? I'm sure a professional in that field would have more ideas.
And I know this isn't what you asked, but losing some weight will have at least as much benefit as bed exercises, probably more. Again, having the input of a professional would be helpful. Folks here are "experts" in DS and other bariatric surgery approaches, but if you haven't (yet!) had bariatric surgery, a dietician could help.
And lastly, I would suggest searching on Youtube for videos that may have other ideas for people who are stuck in bed.