When an electronic market reopens in the morning how are all the bids handled?

When an order is placed, it receives a time stamp. The time stamp determines the priority of an order on the order book. Earlier orders at a given price are executed before later orders at that price. This occurs for buy orders and for sell orders.

It's worth knowing that cancel/replace orders where you lower the number of shares/contracts in an order do not lose their time priority. However, you cannot cancel/replace with a larger number of shares in your order because that effectively let's you cut in line and prevent others from trading.

Between the below 2 questions you will have answers to your question.

If you still have doubts, please ask a specific question.

How are unmarketable market orders (other side of the order book is empty) matched with incoming orders?

How do exchanges match limit orders?