AllCampaignsRemastered - Updated mod for all Campaigns

The question is: are cutscenes in game maps separate maps that are part of a larger map, or not? And if so, how do you open and edit them in the map editor?

If I understand the question correctly, you're talking about in-game videos (dialogues). Separate maps are used for these videos, and they are located in the Maps/SmallSpecialArenas folder. You can edit these maps by creating any new map in the editor, making sure the size is the same as the video map. Save the map, and in the archive of the saved map, replace the Map file with the map you want to edit. You also need to rename it to Map.