Two buttons in the top bar of the review: Request changes and Approve. Both are recorded against the round you are looking at, with who decided and when.
Approving
Press Approve. The version is frozen — no new comments can be added to it — and the review shows as Approved on the project and in the portal. Everyone on the review is told straight away; a decision is the thing people are waiting on, so it never waits for a digest.
Requesting changes
Press Request changes and say why. The round is closed the same way, marked as sent back, and the designer is notified. Upload a new version to open the next round.
Changing your mind
You cannot un-approve a version. Approvals and change requests are the audit trail — they say what was actually signed off on. To review it again, upload a new version; the earlier decision stays on the round it belongs to.
Who can sign off
Your team, and the client on the project through the portal. Somebody using a guest share link can comment but cannot approve — a sign-off has to come from a named person.