In Lightning, the standard lookup field shows five records as we type in search criteria. How are these five records selected to be shown (e.g. most recent, most searched, alphabetical, etc.)?
As per the Salesforce article https://help.salesforce.com/articleView?id=search_lookups_how_works.htm&type=5
As you start typing in a lookup field, the results adjust dynamically with records that have matches in the Name field. Records include recent items, items from your most frequently used objects, and items from the current object. It’s also possible to create a record. A secondary field under the record name displays contextual information, such as the contact’s phone number.
Hopefully this helps.