What are these round pads on the bottom of a PCB?

As mentioned in some of the comments, these are test pads designed for use with a pogo-pin test fixture, also known as a bed-of-nails test fixture. These fixtures have a specially-shaped array of pogo-pins which press down and make contact with the test pads on the board under test:

These test pads and the test points are probably used in different parts of the manufacturing process. For example, the test points may be used for board-level testing and the test pads may be used for system-level testing. Or, perhaps, the test pads are used for programming and/or debugging firmware, when the device under test is clamped into the bed-of-nails fixture.

They're called test points. They're places to stick your multimeter probes, or, more commonly, an industrial test jig.

I think SC could be short for "spring contact" (spring loaded contact), in which case the PCB will be mounted together with some other PCB, similar to a Arduino header, but a different connector type not using male and female pins.