Start here. Once you render a video it's a video. If you want to do more with the composition then you have to open the After Effects Project file you used to create the composition. It's the same way with any video editing app. You render the video and it's done. If you want to edit the pieces you have to open the program used to create the edit and edit it again.
Really basic stuff here. You have to go through the baiscs or you will be wasting your time.
Thank you so much! I have only started the tutorials yesterday. I have a lot more to learn.