Copying animation keyframes from one bone to another with flipping Problem:
I have two hand bones on a character that are meant to do the same animations, but on different sides of the character (left hand and right hand). This is on Blender 2.92.
I've gone ahead and animated from the side, before realising I've only animated one hand. I did not have the other bone rig selected, so the other hand missed out on getting the keyframes.
I would like to copy over the animation keyframes from the left hand over to the right hand, with little to no re-work needed.
What I've tried:
I've tried to copy and paste the keyframes from the animated left hand to the non-animated right hand. It however, does not seem to work.
If the hands are holding something, the alternative would be to make the right hand a child of whatever the left hand is holding via a bone constraint. I'm not seeking to do this though, I would like to have the keyframes available for both for tweaking down the line without messy constraints logic.
Figured out how to mirror the same animation keyframes to the other bone hand, without any re-work or manual labor required.

With the animated hand's bone rig selected, select all the keyframes in the Timeline (Shift+F12 to get the Timeline view and use your mouse to box-select them all or press Ctrl+A) and copying them with Ctrl+C.



Then proceeded to select the other hand's bone rig (the non-animated one). With the non-animated bone, first setup at least one initial keyframe in the timeline. I did this with hitting i to put in an initial keyframe.

Lastly, the trick to get the animation pasted and flipped for this other hand bone is to have the Graph Editor open (Shift+F6) and right click in it and press Paste_Flipped (or alternatively press Shift+Ctrl+V).



Done! Now the other previously non-animated hand bone rig is now animated with the exact same keyframes as the other hand, just flipped so it looks correct!
This was all done whilst in Pose Mode.