Until recently, change ids were entirely local (in the git backend). So conflicts just can’t happen.
Recently jj started including the change id in a header in the git object, so the receiving side can choose to respect/expose them as appropriate. An issue with this is that some git commands don’t guarantee the preservation of headers, so it’s possible to accidentally lose them; git rebase being a prime example.
Generally, when a new change ID is needed, jj generates a random number. The individual commits under each change still use hashed SHAs.
> when I can modify the whole commit
You can't actually modify any commits. A mutable change can be thought of as a subset of immutable git commits with a pointer to the most recent commit.
When you alter a file and jj snapshots, it adds immutable commits to the DAG end, and updates the pointer.
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There's some exceptions, like when importing from git, it will generate new change IDs based on a transform of the git SHAs.
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Side note: as much as I like jj, I admit the change/commit terminology is confusing as hell.
