You can repair a damaged vehicle out in the field. The repair itself costs no money and no supplies. What it costs is a repair kit, the item that does the actual work.
No kit means no repair, even next to a repair structure. Nobody repairs a vehicle bare-handed. General stores and gun dealers sell kits for around $150. A new campaign starts you with one kit, already in the trunk of the car.
A kit used out in the open heals a damaged part to half health at most. That gets a wreck moving again, but it does not make the part new.
Three resistance structures heal a part all the way instead of halfway: the Vehicle Maintenance Ramp, the Garage, and the Helipad. You need range and a kit in hand. The ramp and the garage reach around 12 metres. The helipad reaches around 20 metres. A structure does not care what it fixes, so a car on a helipad heals just as fully as a helicopter.
Nothing on screen marks the edge of a repair zone. Distance is your only guide. Step outside the range and repairs fall back to the half-health limit. Step back inside and they are full again. A ruined structure repairs nothing until someone repairs the structure itself. See Ruins and Repair for how a wrecked structure comes back.