So when I want to add a file to my game’s page, there is a limit to 1gb. My question is, if I use an external link, such as github or google drive, will this increase that file size limit on Itch.io?
...No, it will simply not be taken into account, since if the file is externally hosted, that means it's not on itch.io's servers. Anything you upload to itch.io still has to mind the limit.
