A project in CircuitStudio consists of links to all documents, setups and output files related to a design. A CircuitStudio project file (a simple ASCII text file) lists which documents are in the project and stores project specific options. Typically you would use one folder (directory) per project, the project file and associated documents (schematics, PCB layout, generated outputs) would be stored there.
Not shown on the video but well worth knowing. If you take a look in the folder of an existing project you will also find the History sub-folder, CircuitStudio keeps local backups to help recover an older version of a document should you make a mistake or need to recover an earlier version. You can recover older versions through the CS user interface or, in worst case, directly from the History folder.