More structured runs always have a kit. In the kit you’ll find your Bib with your number. Often there’s something glued behind the bib, it’s a sensor.
In specific places on the route the organizers will ass some sensors on the ground, so when you move across them they’ll register the time.
By going through a few sensors they’ll know when you started and when you finished (with eventual intermediate steps as well).
More structured runs always have a kit. In the kit you’ll find your Bib with your number. Often there’s something glued behind the bib, it’s a sensor.
In specific places on the route the organizers will ass some sensors on the ground, so when you move across them they’ll register the time.
By going through a few sensors they’ll know when you started and when you finished (with eventual intermediate steps as well).