Usually I only have to copy one media file into my NAS folder (which points to Plex app).
But now, I have a few folders for just one movie (it is a full Blu ray movie rip) and it looks like this (second image is the files within BDMV folder).
How could I add these successfully inside a Plex movie folder/ application ?
Use MakeMKV to extract the files from the BD structure.
When you launch MakeMKV, choose File → Open Files, then pick the index.bdmv file.
You will most likely see many files listed. The movie will be the largest one and have multiple chapters. Others are extras, movie studio trailers, movie trailers, etc.