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.