From what I have read and heard the way that this virus is spread is via aerosol transmission and I can not find any definitive studies on the effectiveness of everyday cheap masks for preventing this because as somebody I heard said recently it is like trying to use a chain link fence to keep mosquitoes out. If such a study exists please send me the link anybody.
My understanding is that whilst the virus is significantly smaller than the mask holes, the moisture droplets that carry it are mostly caught by the mask. This is why the mask gets damp inside. So they are most effective at preventing the wearer spreading what they might have.
Generally, when speaking, exhaled droplets will fall within 1.5m, hence the 2m rule. When speaking more loudly, shouting, or singing, the droplets are exhaled faster, and can dry out in flight, leaving the virus airborne. This is why gatherings like parties and weddings were restricted, as noise levels cause people to shout, as do night clubs, etc. You'd have thought that cinemas could have stayed open, aren't people supposed to be quiet there?
If the mask is used for some time, or when using a degree of effort, it can get quite wet inside. At that time it should be replaced, and the used one disposed of carefully, or washed. I think we were supposed to carry plastic bags to put them in. (Like a doggy-doo bag, but without the need for a dog.)
If the outside of the mask is touched, (none of us do that do we?), the moisture can be forced/drawn through the mask, allowing the virus, if present, to be expelled. Also, once the barrier is breached in this way, virus particles landing on the outside, can transfer to the inside via the moisture, to be inhaled by the wearer. (Like a tent that keeps the rain out, until you touch it, then the water passes through becuase the sufrace tension is broken.)
There appears to have been almost no flu cases last winter, suggesting that mask wearing does have some effect.