[QUESTION] How does Git ref backend work

1. I noticed that normally refs transactions use the refs files-backend,
looking at the reference transaction of refs files-backend alone:
First, it opens a transaction in memory, then multiple update items
are added in memory, and finally, when the reference transaction
is committed, it creates a ref.lock for each update item, renames it
to the real address of the ref.

but there seems to be some packed-backend logic coupled inside of files-backend. This makes me wonder why these ref backends are not independent?

2. Is the reftable backend in a usable state? How can I enable it?
I couldn't find any clues on https://www.git-scm.com/docs/reftable.
3. Will future git refs backend possibly support external storage media
e.g. Redis?

Thanks, -- ZheNing Hu