No, more like:

    git rebase -i
    # squash all the commits (e.g. in vim with ctrl-v)
    git reset HEAD^
    git add -p
    # interactively pickup the RED hunks
    git ci -m RED

Definitely not. Switch to a previous commit, make edits, changes propagate into the future commits (including into a git repo if you wish [1])

Jj is not git and is not a git tool, it just (thankfully) uses git as a backend, so you can still carry on with the rest of the world.

[1] https://news.ycombinator.com/item?id=47765759

I'd recommend reading the post, it's not that long

git rebase -i kinda sucks once you tried jj.