How can a lock manufacturing factory produce several locks and each different from all others?

Answer 1

The most common type of lock today has a cylinder and 4 to 6 tumblers (Yale lock). Each tumbler can be made in one of 250 different lengths.
If you have a math background you can figure out the number of combinations possible - and it is huge.

Answer 2

Inside each lock are cylinders. There are two little spacers in each one. There are around 5 cylinders in each lock. So by changing the length of the pins in the cylinder, the key has to be different to accommodate the different combinations of pins in the cylinders.