The walk up experience online checking defaults to a location, as described in these community posts:
Some users may wish to have a different behavior.
Server Code on the Widget "Online check-in Experience".
The selection of queue that is physically nearest to the user is calculated based on latitute/longitude co-ordinates. So in sys_user table, user has a location assigned and location(cmn_location) table has those co-ordinates defined. The logic for how walk-up application is fetching the nearest location can be found in sp_widget_f84cfc133b7303001d132c2b54efc476.
There is a function in the server script called getClosestWalkUpLocation() that is invoked to get the location that is closest to the user. If no lat/long co-ordinates are found on sys_user record, the user will need to select the location manually from the dropdown.
You can customize the widget to suit your needs.