Compositions and footage items are listed in the Project panel. Unlike items in the Timeline panel and Effect Controls panel, the order of items in the Project panel has no influence on the appearance of the movies that you create. You can organize footage items and compositions however you like, including organizing them using folders. Solid-color footage items are automatically placed in the Solids folder.
Folders that you create in the Project panel exist only in the Project panel. You can expand a folder to reveal its contents, and put folders inside other folders. To move a file or folder to the top level of the Project panel, drag it to the gray information area at the top of the panel.
You can use the search field in the Project panel to find footage items that meet various criteria, such as those with missing source files. See Search and filter in the Timeline, Project, and Effects & Presets panels.
For a helpful video tutorial about organizing assets in the Project panel, see this video tutorial by Jeff Sengstack and Infinite Skills.