CDC from the Mayo Clinic:
https://www.mayoclinic.org/diseases-conditions/coronavirus/in-depth/coronavirus-mask/art-20485449
The CDC updated its guidance to recommend widespread use of simple cloth face coverings to help prevent transmission of COVID-19 by people who have the virus but don't know it.
The Atlantic:
https://www.theatlantic.com/health/archive/2020/04/dont-wear-mask-yourself/610336/
The good news is that preventing transmission to others through egress is relatively easy. It’s like stopping gushing water from a hose right at the source, by turning off the faucet, compared with the difficulty of trying to catch all the drops of water after we’ve pointed the hose up and they’ve flown everywhere.
Research shows that even a cotton mask dramatically reduces the number of virus particles emitted from our mouths—
by as much as 99 percent. This reduction provides two huge benefits. Fewer virus particles mean that people have a better chance of avoiding infection, and if they are infected,
the lower viral-exposure load may give them a better
chance of contracting only
a mild illness.
And another:
https://patient.info/news-and-features/covid-19-do-you-need-to-wear-a-mask-to-avoid-coronavirus
Self-protection
For most of the general public, wearing a face mask offers very little protection from coronavirus even if supplies of surgical (rather than FFP3) masks were available.
[...]
Protecting others
The main reason for members of the public wearing masks, particularly non-surgical masks, would be to reduce the risk of passing coronavirus to someone else. That's why the German government and the CDC are telling people to use masks when they go out, as people who are asymptomatic or pre-symptomatic might not realise they are infected and spreading the virus.
s.
This was my Google search:
https://www.google.com/search?q=how+masks+protect+others
I tried to link the most recent entries, but there's plenty more there.