these are drag and drop targets, small squares that are uniformly styled to give the user a hint that dropping something here is useful.

Something similar exists in macOS, but isn't widely used, as far as I can tell.

You can create a script in Automator that does things with an input file, and then save it as a desktop icon that you can drop things onto. I have a few of these for auto-resizing images.

(Bonus: Because it's done in Automator, you can also have the same script appear under Quick Actions when you Option-click the file/s.)

Panic's Transmit allows you to create a desktop icon that sends whatever's dropped on it to a server via FTP, SFTP, S3, Google Drive, or a dozen other methods.