Car keys will only grant access to a specific vehicle. They are required to start the engine, unlock doors, unlock the trunk and use the accessories on the dashboard.
When finding a key it is not necessary to equip it, just to be in the proximity of vehicle to which it belongs.
They can found:
On the ground in parking lots near the vehicle.
Inside storage containers in nearby houses.
Inside the vehicle in the ignition, glove box and car set.
A key icon will appear over your character if they are in close proximity to the vehicle while possessing its key. This indicates that it can be accessed and driven. To do so, press the E key or use the vehicle control with the V key.