Smart folders in macOS is a tool that helps you organize and find your files. They are not folders, because they do not contain anything. They are saved searches and can save a lot of time for searches you often perform.
For example, if you open your download folder and sort by Date added to find your latest downloads, you can instead make a smart Folder called "Latest Downloads" that only contains files in the Downloads folder that is added today, and saves you a few clicks each time.
You do them by defining the search criteria (ie all audio files on your device) and they save them as a smart folder you can put on the desktop or in the Finder sidebar. They use Finder's powerful search features, which are often overlooked to write the name in Spotlight.
Creating a Smart Folder
You can find the option to create smart folders under "File" in the Finder. You can get Finder to appear in the top menu bar by quickly clicking on the desktop.
This will provide a familiar search dialog. You can click the + button to the right to add new search criteria.
You can add as many smart folders as you like, and by default, Finder will only pull out files that match all the options you defined. If you want to change this behavior, hold down Options and click the + button, which changes to three points. This will group a list of criteria, and you can choose between All, All and None.
You are not only limited to the default search options, if you select "Other" from below, a complete list of all hidden options is displayed.
Many of these are hidden for a reason, because you will probably never search for an image based on the geotag's latitude, but you can find some interesting options here.
If you are satisfied with your folder, you can press "Save" next to the + button.
By default, it is saved to a special folder "Saved searches" and adds to the sidebar. If you prefer it on the desktop, you can change this.
Some Useful Smart Folders
If you have trouble thinking about ways you can use Smart Folders, here are some of our favorites to get started.
Find large files
A useful search is "File size greater than 1 GB", or whatever you think is too large. Save this search as a smart folder and you can quickly find large files you might want to delete.
Delete Duplicate Files
Because macOS adds "(1)" to duplicate file names automatically, this (along with some higher numbers) searches all duplicate files on your device.
DMGs you need to delete
When downloading a macOS app from the internet, it usually comes in a .dmg file. These files take up space and often disturb your download folder. You can find them all by searching for the .dmg extension. It's probably safe to delete them all, provided you have already installed the programs inside.
The previous example is a bit unintentional to set; You must click "New Smart Folder" from the Downloads folder, which allows you to search for "Downloads". From there, you can filter for items created in recent days.