Masks are Fantastic
Masks, in an ideal setting, produce an immense reduction in the amount of virus that passes through them, on the order of 98-99%. https://pubmed.ncbi.nlm.nih.gov/33441205/ The major issues are people not wearing them properly and people wearing poor ones, which didn't matter much early in the pandemic, where surgical or even cloth masks sufficed. https://pubmed.ncbi.nlm.nih.gov/32673300/ More infectious strains change the calculus because either a smaller number of virions is sufficient for infection or there are just more virions being expelled, and masks are a percentage reduction. A population educated on how to do a proper fit test with an N95 (or KN95) mask should see a dramatic decrease in transmission even with Alpha or Delta, although preliminary data out of Israel, where masking is mandatory indoors and outdoors, indicates that compliance or proper masking is not comprehensive or that surgical masks might not be effective enough anymore. Here is a great review