I have a "Projects" list in ANOTHER SITE and need to get the other fields that have relationship with my "Projects" list on Nintex Forms. I performed a test and it works on environment on-prem. Is it possible in Nintex Forms for SharePoint Online?
You should be able to use the Office 365 update items action to locate the item you want to update in the site and push the content to that item without a problem. Use the query builder to locate your specific item to update.