So I was having a problem in Blender 2.9, where while I was trying to parent the stems and the lower arms to the appropriate bones, when I went into pose mode, and tried to move/rotate the top bone (lower arm bone), the entire model would move.
I searched here and thanks to DustinWoods, I was able to get passed this issue. I tried selecting the objects, and then the armature, and then went to edit mode, selected which bone I wanted to parent to, and within edit mode, hit CTRL + P and in the Make Parent menu, selected ‘Connected’, after which I was able to move the lower arm (using the armature) in pose mode.
Thanks DustinWoods and hope this helps.