For Microsoft Flight Simulator 2020 you can presently choose from 3 suppliers of AI aircraft: FSLTL (FS Live Traffic Liveries), AIG (Alpha India Group) and Multiplayer Traffic Library (MTL).
FSLTL - FS Live Traffic Liveries
FSLTL AI aircraft can be downloaded and installed through the FlyByWire Installer only. More information about the installation and a list of all available liveries in the FSLTL library can be found at the homepage of FSLTL. You do not need to install the "injector".
AIG - Alpha India Group
The official way to get AIG models is by downloading their AI Manager and then select and fetch all the models you like.
Make sure all AIG aircraft will be installed in the
Community folder of MSFS 2020.
Multiplayer Traffic Library (MTL)
MTL models are not as pretty as FSLTL and AIG, but in the MTL you may some aircraft types that are not featured in the other two collections. To access the MTL database, use the MTL Installer. This is a rather new program and if you have old MTL models for MSFS then you'll have to delete them first. Follow all the instructions on their website!
If you prefer first checking what models they offer, check out their catalogue of aircraft.