I've tried installing the First AIR Instruments for Pro Tools First 2020.9, and while they make it to the Program Files/Common Files/Avid/... plugins folder, I only see them in the "Recall Inserts" section in the Inserts popup menu.
There was a similar issue with trying to install Spitfire LABS free instrument plugins, but I read somewhere that newer versions of Pro Tools might not allow any-old third-party .aaxplugins without some sort of license handshake so it knows they are legit? LABS appears to only be for the full version of Pro Tools according to tech support.
I also tried an older version (10) of the AIR instruments plugin, but that didn't even put the plugins in the plugins folder.
I downloaded the latest First AIR instruments plugin from here, I don't see it in the marketplace:
https://www.avid.com/plugins/first-a...trument-bundle