Vast majority of third party content (aircraft, skins/repaints, scenery, airports etc.) go to the community folder. The game should give you prompt to choose the directory, but by default it would be something akin to C:\Users\yourusername\AppData\Roaming\Microsoft Flight Simulator\Packages/Community.
If you are downloading an actual freeware aircraft from Flightsim.to, say the RV-7, then you can just drag and drop the uncompressed folder into your community one (so that the layout and manifest files are in the folder below the community one). The payware products from Just Flight’s online store (such as the Piper Warrior) use an installer to create the aircraft folder into your community one.