Hi, @BM136 .
The address used in the customer’s latest order will be used as a default address.
So if at some point a customer uses a new address instead of choosing one from their saved addresses list, this new address will become their default address.
Keep in mind also that a customer can select the Save this information for next time option during checkout. When this option is selected, it’s the customer’s browser that will save their contact and shipping address details, not Shopify. So the next time they shop from your store using the same browser, their browser will automatically fill in those details with the cached information. More information on this can be found in this guide.