When a Service Resource starts to work they use the Check In action on a Service Appointment, the details in the screenshot below are captured on a Service Appointment record.
In this example the Service Resource (Egor Burya) we can see:
Check In is clicked from the FSL mobile app at 6:51AM
The Service Resource last known geolocation has been copied from Service Resource record to Check In Location field on the Service Appointment.
Based on the last Check In Location Date field we can see this geolocation has been updated 2 minutes before Check In is clicked.
Also the Actual Start date/time field has been updated automatically.