I am very new to portal speedrunning. How do I compile my demos into one video that I can then view and/or upload to youtube so I can have a video link when I submit a run?

You can just submit the demos as long as the run isn’t paused and/or in the top 10.

Do not put the link to the demos in the video link section if you are submitting with demos only. Put the link in the description and leave the video link section blank.

This is the submission guide that is in the guides section: