No vehicle lasts forever. Unless a vehicle is a beautiful vintage car, it is almost certainly destined to be scrapped one day. Typically, once a vehicle has reached the end of its useful life, it is taken to a junk yard. Junk yards often keep vehicles for a while after they purchase them. This allows them to earn money off of specific parts that are valuable to mechanics and people trying to extend the lives of their own vehicles.
Once a car has been in a junk yard for a while and is no longer useful even to mechanics, it is destined to be recycled. Many junk yards also function as auto recycling services. They take the old cars, which have already been drained of all fluids and have had their batteries removed, and they crush them down. Then, they shred the vehicles and remove any non-metal parts they can easily recover.
Once the cars are reduced to crushed shells, they are taken to a facility where they are melted down. The molten metal can then be separated into steel and other metals to refine it. Typically, the metal is forged in to tubes to be used by a variety of industrial businesses.