Two events, one word
A manual action is a human decision against a specific policy, recorded in the search console's manual actions report and accompanied by a message. It is applied deliberately and can be revoked after a review request.
An algorithmic reassessment is not an event at all. It is the continuous output of quality systems responding to the site's pages, and it produces gradual or date-aligned declines with no message and no report entry.
People say penalty for both, then look for a rule that was broken. If the report is empty, no rule was broken. Something about the pages changed the way they are judged, or something about the competition did.
Get this distinction right first. It decides whether your next three months are spent on a review request or on the pages themselves.