That's the file structure of a Blu-ray disc that's been unencrypted. Get MakeMKV and open the main directory using that software. It will read it as if you've given it an actual Blu-ray disc. It will show you the available video files. The largest one will be the movie, so if you want, you can select only that from the list and it will rip it out of the file structure and give you an mkv file of the movie. It'll be large because it's not compressed, so to save space, you can use Handbrake or ffmpeg if you're nice with command lines to encode it to your favorite codec.

If it's a movie blu ray you can usually play the "index.bdmv" file in a compatible media player e.g. VLC definitely works. MPC-HC / MPC-BE works too, I think(?) MPV can play them too. As well as Jellyfin and Kodi if that's your thing.

Alternatively browse into the "STREAM" folder, usually the biggest .m2ts file in there is the actual movie or whatever it is you want to play. The above media players can play that directly if preferred.

For TV series the above usually works too but the episodes are usually split out among multiple .m2ts files so it might be easier just to play them directly in that case.

that's the structure of a bluray disk. It includes all that the player needs to play it (e.g. menus, chapters, etc). The STREAM folder contains the actual video, probably in a .m2ts file (you can convert it to mkv using ffmpeg or similar tools)

@Joseph_Boom I just go to the biggest file, and it is usually the movie