Nowadays, if your cell phone has enough memory, you can install a lot of software.
You can buy her a smartphone with a big screen and a lot of memory, help her install all the software, and then teach her how to use them one by one, and you can also handwrite them on paper so she doesn't forget the steps.
Again, there are so many short-video programs that can be ****ed up with each other, and it's not like they installed one and the other one can't be used, so install both Shutterbugs and Jitterbugs, and see which one she prefers to use.
The most important thing is that usually also use these video software and grandma video call a little, see what she needs, lest she lonely, this is much stronger than let her watch short videos alone.