The phrase is often used to mean somebody is watching. That is not the same thing. A human in the loop has a specific decision to make at a specific step and the authority to reject, which means the workflow has to be built to stop and wait.
The two places it always sits are the brief and the final cut. Everything between those can be automated, batched and run overnight. Those two cannot, at any volume, for any client.
QUESTIONS PEOPLE ASK
Can AI content production be fully automated?
The production can. The judgement cannot, and should not be. Automate the drafting, the variants and the formats; keep a person on the brief and the final approval.