I am preparing photos for Adobe Stock guidelines, my first upload. I have these photos in a separate collection. If I edit these photos will the changes apply to the same photos existing in other collections?
Yes, collections are just different 'places' that the same photo shows up. So when you edit that photo, the new edits are seen in all the collections it belongs to - as well as, when it is viewed in the context of its folder on disk.
Unless: you make separate virtual copies of this photo, These virtual copies are independently editable, independently keywordable etc, and can be independently added or not added to Collections. When you go to view that folder on disk (inside LrC) all of these virtual copies will show: they all refer to (share the use of) an imported file which lives in that folder.