You are correct that to some extent the employment figures are like car mileage figures, in that much of their use comes in comparisons, not hard numbers.

The BLS explains here how they get the two basic numbers, the non-farm payroll employment numbers and the household numbers.

One point of confusion is that articles that count the number of jobs created are almost always counting the payroll number (so I do too), but the unemployment rate is calculated on the household numbers. There have been many months where the two job numbers are wildly different. I have no idea why.

Obviously, the unemployment rate is already calculated as the ratio of people unemployed to people who are employable, so it automatically accounts for the overall universe of workers. It's why the number remains fairly constant although millions of new jobs are being created--the population of workers goes up just as fast.