Question about proportional voting

Currently, we vote for one representative per riding. The issue with that is that (hypothetically) one riding could have a million people and another could have 100 people. But both of them would have the same amount of power in Parliament because 1 riding = 1 mp.

How would that work in a proportional election system? Is there one candidate per X number of citizens in an area? Wouldn't cities be over represented? Wouldn't there be one candidate to cover very large sparsely inhabited areas that might not have the same needs from one spot to another?

I'm really curious how this would be implemented.