At one time print production took place directly from the negative. Either a contact print the same size as the negative or an elarged print from basically a vertical projector called an enlarger. Digital technology has come to the aid of the analogue photographer and it is much easier to scan a negative and process electronically in software.
Specialist scanners are available but it is also possible to photograph a slide or negative against a back light with a digital camera. In fact many of the cheaper digital scanners are in fact just digital camera set ups.