How do I find the buyer of an order?

Short answer: In the order's details, the Customer section gives the buyer's name: click it to open their contact sheet. The envelope icon beside their email address copies it in one click.

Steps

On the Orders page, click the order's row.
In the Customer section, click the buyer's name: their sheet opens on the Contacts page.
To copy their email address without leaving the order, click the small envelope in front of the address: it goes to the clipboard.
To write to them, go through their contact sheet: sending an email always starts from the Contacts page.

You'll know it worked when: The buyer's contact sheet is open, with their history and their details.

Their other orders

To the right of the email address sit the numbers of the same buyer's other orders. Each one opens the matching order — handy when someone writes to you about "my order" without saying which.

The route back

From a contact sheet, the Order Summary section lists their orders with their date and number. Clicking an Order # number brings you back here, to the order's details.

An order can have no buyer attached — the section then shows No customer linked. That happens on very old orders, placed before the contact existed in the dashboard.

Going further

What is in an order's details
How do I send an email to my contacts — writing to a buyer from their sheet.