The easiest way to find it for your personal installation is to open the sim, go to the Marketplace page and click on the “Library” tab at top left. From the next screen, you will see a search box and a Settings (gear) icon to the right of the box. Click that gear icon and the popup will show you where the sim has put the Community folder. You can open that folder using a button on the screen. You can then pin it or create a desktop shortcut to it for later use.
ALTERNATIVELY: from that same screen you can navigate to a folder of your choice and tell the sim to create a new Community folder there. IMPORTANT NOTE HERE: this location will be the PARENT folder of where you want the new Community folder. In my case, I have my sim installed to E:\FS2024, which is where I want my custom Community folder to live. So that is the file path I used in this dialog box in the sim. Therefore, my new Community folder is located at E:\FS2024\Community. Once you quit the sim and reload, anything you have placed into this new Community folder will be available in the sim. However, currently the Library screen itself in the sim is bugged and you will not see these items on that page. But they WILL be available in the sim: airplanes can be selected from the Free Flight page, any airports will be visible on the world map and will be rendered in when you fly there, etc.