For the Dataverse, they don't show up on the list of available entities to look for CRUD triggers on.
So the need is this. I have "Document" records that are linked to Products and Contacts. A Document record can be linked to numerous Contact records; a Contact record can be linked to numerous Document records, thus the many-to-many entity.
There is a tab in the Contact record's form that lists the corresponding Document records. The Document associations can be assigned and removed from this tab.
However, I'm finding this does not trigger an "Update" on the Contact record even after adding/removing the Document record association and clicking "Save."
This makes sense I suppose since the update is happening in the many-to-many entity and not the Contact. Again though, these many-to-many entities don't appear in the list of entities to watch for CRUD operations on.