TVDB Collections are setup for not only TV + Movies, but also for Movies with Intended Sequels. So it’s very common to have 1 movie in a collection if either of these is the case. For instance, Avatar currently only has 1 movie in the collection, but there are 4 other movies coming out between now and 2027 or something ridiculous like that. Or 2001: A Space Odyssey also includes 2010 : The Year we Made contact.
If you want the folder for the collections, leave it the way you have it. If you don’t want the folder for the collection, remove the {collection} tag.
Regardless of which you choose, Plex will still place them in collections if you have that option set in the Agent. It does not function off of the folder structure.
TL;DR: file naming and folder structure has no influence over collections inside Plex.
These are either fetched from TMDB by the Plex agent (can be switched off)
or
are read from the embedded meta tags of your mp4 files. If you have a video file.mp4 which has the ‘Album’ metatags populated, then Plex will put the video into a collection, named after the content of the Album metatag.