If a user of the platform wants an accountant to actually review their tax return, how does that engagement happen?

The engagement does not happen on the platform itself. If someone using the tool wants their return reviewed, the platform directs them to a local verified accountant/firm associated with that jurisdiction (e.g., 'in Cyprus we know these guys, send them an email'). Those accountants did the groundwork of reviewing and verifying the skills for that jurisdiction. It is then up to the individual firm whether they want to take on that person as a client, typically via a normal engagement letter process, not through the platform.