jj has two kinds of these logs: the evolog and the op log.
The git reflog is based on, well, refs. Whenever a ref is updated, you get an entry. This log is per ref. That's HEAD, your branches, your tags, and your stash.
jj's evolog is sorta similar, but also different: it's a log, but per change (think commit in git). This means it is broader than per ref, as it includes not just commits that correspond to a ref, but all of them.
jj's oplog is a log per repository. This lets you do things like `jj undo`, which lets you get the entire repository, not just one ref or commit, back to the previous state, easily.
But instead of commits, most of the time you're working with (the not greatly-named) "changes", which are a conceptual history of related commits. E.g., a single change ID points to the most recent commit in a list.
As long as you keep editing on a change, it keeps accumulating commits under the hood. When you move to another change (via `jj commit`, `jj new`, etc), all your new commits end up under a different change.
What happens when you merge or split changes with the change history? It sounds a bit like applying a VCS on top of a VCS.
I'm not sure what happens, but I suspect a merge would create a single merge commit with the tips of the parents' histories, and then add new commits from there on.
A split is probably conceptually similar to making two new branches, except each of the new commits gets one of the branch tips. Really just guessing here, I have no idea if that's how it works, or if they share commits or not.
Merging is making a change that has more than one parent. You can then resolve any conflicts within that change.
Splitting a change into two will update one commit with one half and make a new change with the other half.
With jj you just "jj op undo <operation_id_that_fucked_your_repo>" and you're fine.
Editing prior commits is also pretty easy, which in turn makes fixing merge conflicts pretty easy too.