They were built by the Lotus factory in period, and raced by proud owners until they became uncompetitive.
When historic motor racing began these cars were unearthed form barns and the backs of garages, restored and raced again.
The pictures you show are the Formula Junior cars, and Duncan Rabagliati who runs a world wide series of FJ races has been the main driving force to ensure that so many have survived.
The sports cars (which you did not show) have been championed by the Historic Lotus Register (founded in 1974 by Victor Thomas) which keeps records of all the early race cars. It is a club with its own website which you might care to visit Historic Lotus Register - All about Colin Chapman's early Lotus cars. There is also a house magazine called "Historic Lotus" which covers both two seat and single seat models.
It is fantastic to think these cars are still used. Living over here in California we get to see all sorts of exotic cars but never a collection like this. I made a trip to the British GP this year and saw this car (not going anywhere as it had no drive shafts!) and the closest I will get to owning one is when my little 'project' is complete.