Depending on the application you are using saving your project file will be different. Garageband, for example, automatically saves your project file for applications like Audacity you will manually need to navigate to File > Save.
If your application has an auto-save feature (usually found in the properties menus of an application), it is recommended to turn it on to auto-save every 15 - 25 minutes.
Saving your project file will allow you to continue editing your project at a later date.
Every platform will have a different way to share or export your video project. If given the option, make sure your export settings match your timeline/project settings. You will likely need to balance quality with file size since uploading video files can be a strain if your internet connection is weak or unreliable. The smaller the file size, the more compressed the video and the greater chances of unwanted artifacts interfering with your video.