They already do explain this on their Airtable documentation page.
A Date Created field triggers for newly-created records.
A Date Created field will never re-trigger any automations because there can only be one created date per record that never changes. The creation date of a record never changes.
No. You have to control this on the Airtable side, not on the Make side. Make your date modified field change based on the fields that you are monitoring.
Alternatively, as I mentioned above, just trigger everything based on a webhook, which might be easier for you to setup & maintain.
You may also want to go through the Certified Make Training Courses, which I think are extremely educational.