AMO orders (Limit and Market) in the equity segment are sent to the exchange at 9 AM, during the pre-market session. More details here.
During pre-market session from 9 AM till 9:08 AM you can only place, modify or cancel orders.
After price discovery is complete, there is an order matching period from 9:08 AM till 9:12 AM. Orders are matched at the equilibrium price, where Limit Orders are matched with Limit Orders and Market Orders are matched with Market Orders. You can check how the pre-market session works here.
Orders which aren’t matched are carried forward to normal trading hours. Also, unmatched market orders in the pre-open will be carried over to the normal market with as limit orders at the equilibrium price, explained here.