I broke my arm some years ago, and was in various sorts of casts for the better part of 3 months.
The hospital gave me something that made getting the lids off of jars fairly easy. It was just a square of lightweight rubber stuff. I had an electric can opener, though, over here, most cans have ring tops now, that just pull off.
(I LIVED on canned beans when I broke my arm. It was easy to open the can and pour them into a pot.)

Now: To medicate a cat one handed, as I had to do.
1. Catch the cat napping (That isn't hard.)
2. Drop a blanket over cat.
3. Wrap legs and blanket around cat.
4. With functional hand pry open cat's mouth.
5. With semi functional hand squirt medicine into cat's mouth.
6. With functional hand hold cat's mouth shut until cat has swallowed medicine.

One handed life was so complicated, I aquired a housemate. He stayed with me for the next 8 years.