Based on reading between the lines from what experts say, and verified by speaking with a biomedical researcher friend:

==> the density of virus you're exposed to and how long you're exposed is what makes the difference in whether you get infected.

Distance is not what matters most.

It's the density of the virus in the air that you breathe and how long you breathe it that matters.

If you're in a confined space that's virus-rich but only there for a short time, the risk is relatively low.

A supermarket clerk is not at high risk, because people come through the store quickly, in and out in 15 minutes. They breathe virus in the air, but the density is a function of how many people who come in the story who are infected.

The worst case: a choir in a small space over a long time with an infected person. Singing increases the volume of virus that's projected into the air.

People in factories are at relatively high risk because they're in a confined space with the same people for a long time.

The more people who are infected, the longer the time, the more risk.

People behind the counter, segregated from customers, as in a pharmacy are at high risk if one or more is infected. Small volume of air over a long time.

Outdoors you're relatively very safe because the volume of air is effectively infinite. The density over time can't be high, because the virus dissipates.

In math terms, it's the area under the curve and the density of the virus that matters.