Saving ".prel"s

Or are you saying that when you reopen the project, there is nothing in the timeline?

Adding to John's reply, consider that a video project is a collection of many files.  The project itself contains scratch disks, previews, titles, transitions and other files that are the project.  It also includes the source video, photo and audio files.   Once a project is started all of those components have to remain where they are relative to each other for the project to work.  As John writes, you use the project to create the output video.  You will recognize it in your files as one that ends in ".MP4", not ".prel".

A project file is a command file with pointers to your video files, and instructions on what to DO to those files

When you want to create a new video file you use the share/export function

http://www.pacifier.com/~jtsmith/PrE2021.txt
- As of 07-05-21 my notes file contains 30+ links to YouTube tutorials
- Rather than taking up a long forum page, you can click my link and look
- Plain text file so you will have to copy-paste the links into your web browser