The most important study on masking:

https://doi.org/10.1002/14651858.CD006207.pub5

Title: Physical interventions to interrupt or reduce the spread of respiratory viruses

>There is uncertainty about the effects of face masks…The pooled results of randomised trials did not show a clear reduction in respiratory viral infection with the use of medical/surgical masks during seasonal influenza. There were no clear differences between the use of medical/surgical masks compared with N95/P2 respirators in healthcare workers when used in routine care to reduce respiratory viral infection. Hand hygiene is likely to modestly reduce the burden of respiratory illness. Harms associated with physical interventions were under‐investigated.

This systematic review is comprehensive and rigorous, excluding findings from observational studies and only including results from randomized controlled trials. Bear in mind: this review covers ALL past "mask research," and it contains answers to most of OP's questions.