Anyone can. In your dashboard, open Your services and switch on your free intro. Your checklist there, Booked from your profile, shows what a booking needs: a photo, a headline, your location, your public link, your weekly hours and a connected Google Calendar (it makes the Meet link and checks for clashes). Until it is all done the intro stays off; with no weekly hours it switches itself off. The free intro needs no review.
Once you are bookable, people can book you from your profile and in your communities, and reviews are on from your first session.
Other sessions go through Experts. Adding a session besides the intro, free or paid, asks to list you in Experts: it switches on List me in Experts and sends your profile to the openhour team for review. Those sessions open to bookings once you are approved; your intro stays bookable meanwhile.
Inside a community, its admins decide. openhour's approval counts on openhour only. On a community's page, Offer sessions in … asks its admins to approve you there; until then your free intro can still be booked there.