They record what the miles are upon delivery. For example, if you are picking up your car at the delivery center, they will record what the milage is from that point. If they deliver your car to your home, which is 97 miles away, they will record the mileage from the time you accept delivery...so if they drove it to your house, it will likely be just over 100 miles, but your clock would start from there.
I highly recommend you bring an inspection checklist with you when you take delivery. Throughly check the exterior's panels (one at a time), the lines, the weatherstriping (one of mine was missing at the trunk area), the paint, the headlights (mine had a rock chip), tires, wheel scuffs, the undercarriage. Check the interior paint when you open the doors, trunk, frunk, charging port. Make sure the doors and windows all open and close with ease. Check the interior stitching on the seats, etc. Spend lots of time doing your inspection, then address it right away with the service center before you take delivery since you're there already. They may be able to fix right away, or at least schedule it for you. Make sure to take pictures of every issue and upload it to your service request through your app if they can't get it fixed while you're there. It's all about documenting the issues. While you drive your car within the first 100 miles, listen for any issues...wind noise, etc...check your audio system...check your AC and heater, heated grips, etc...make sure everything works.
Good luck to you!!