New runner here.

I have done a run and have the demo ready to go but have to clue how to upload it to the run submission form. On my first run I used mediafire but I'm not sure if that's the mose effecient way for the mods. Advice would be appreciated.

Upload your demos to a file storage platform (Google Drive, Dropbox, OneDrive, etc.), share the file, get a link and paste it into the description field.

I usually compress them into an 7z file (zip works too) and just put them on google drive and put them in either the video link section or the description

don't put them on the video link, put them on the description. it's hard enough as it is to find runs with actual videos