Why did plague doctors wear those masks with the beaks, and how effective were their uniforms at protecting them from the plague?

There will never be a 'final' answer on this subreddit, because there can always be something new to the table. So if YOU reading this, (yes, YOU) can contribute further regarding matters of plague and disease and the relevant masks, don't let this post stop you from writing!

And while we wait for anything further, OP, you may be interested in these previous threads:

u/Sir_David_S expands on miasma theory, which is relevant to the masks
but as for the specific bird format, u/BedsideRounds has looked into them, and so has u/DoctorVictorShelley, who actually answered their own question after research.