I use Premier Elements 19 on my Windows 10. This is the first time I've created a video and read that it's best to save the video/movie as an ISO file on my hard drive. Right now, it's an MP4 file. How do I save the movie as an ISO file?
Where did you read that? The only reason that I know of to save an ISO format is if your intent is to make DVDs. By today's standards, that is a very low quality.
The project itself won't go away unless you remove it, delete the media files or move the media files. MP4 (with an H.264 codec inside) may be the most universal format for sharing your completed video.