Masks probably work, if everyone follows the instructions to the letter

Not a chance.

Surgical masks are not designed to prevent viruses getting through. They don't even prevent smoke particles getting through, and they are tens of times larger than viruses. The smallest smoke particles are typically 2.5 micrometres in diameter. Covid virions are typically 60-140 nanometres in diameter so the biggest Covid virus particles are nearly 20 times smaller than the smallest smoke particles. Trying to block airborne viruses like Covid with a surgical mask or a cloth mask is like trying to block mosquitoes with a tennis net.

So forget surgical masks. They're designed to stop droplets, not airborne viruses, and frankly even in the limited use of surgery under controlled conditions, there is no evidence that they reduce or prevent infections (citations supplied on request). Surgical masks, even in surgery, have no discernible effect on infection rates.

N95 masks are better, and if used correctly and tightly fitted, they may even be moderately effective for an hour or two. I say may because there is no good evidence that they actually are effective in the real-world, but theoretically based on laboratory tests where the edges of the mask are glued down over an air intake, so as to give a perfect and permanent fit, they seem to block virus-sized particles.

So I'll allow that, if N95 masks are used perfectly, they might block some percentage of airborne viruses for a limited time. Whether that corresponds to a lower risk of infection is another question: theoretically, even a single virus getting through could be enough to infect you. In practice, that's not likely, but if we're going to accept that masks theoretically block viruses under perfect conditions when we know that in practice the conditions are never perfect, then it's only right to also accept that theoretically even a single virus getting through could infect you.

So the big question is, even if you use a N95 surgical mask perfectly, and it blocks (say) 90% of virus particles, does that reduce the risk and severity of infection? Very likely not by much. Look at it this way: suppose somebody is firing a machine gun straight at you. If you block 90% of the bullets, you'll still be just as dead as if you blocked 10% of the bullets.

(The analogy with a virus with a 99.99% survival rate is not perfect.)

There's another catch. When you inhale, any viruses in the air you breath are just as likely to be exhaled again before they can latch onto cells in your nose or lungs and infect them. But if you are wearing a mask, some percentage of those exhaled virus particles will be caught in the mask, where you are likely to re-inhale them on the next breath, giving them a second chance to latch onto a cell. We know that masks can become contaminated with bacteria and fungi, and that you can then breathe those bacteria in from your own mask. So it's theoretically possible that wearing a mask for long periods of time might increase your chance of viral infection. Which might explain these results.

CC u/Zommy