Save recording as WebM video file to disk
Use the "Save to Disk" button in Screencastify to export a WebM video file.
This file can be played back with many video players like VLC, or by just Drag&Dropping it on Chrome or Firefox. You can also use this file to upload it manually to video hosting sites.
Tip: it's usually more convenient to use the integrated upload to Youtube or Drive and just share the resulting link.