@JocelynD said:

Hello,
I am a student and for my final project (a fake e-commerce website project that is not intended to be put online),
I want to retrieve movie data to create products (DVDs) in my database.
I would like to retrieve movie data using collections large enough to have a substantial sample of products.
I would like to retrieve a list of existing collections, but I haven't found the query to do so.

I don't know if I understood correctly, but...

See if this data from a Collection is the data you need.
For example Star Wars Collection
https://www.themoviedb.org/collection/10-star-wars-collection
Using this API Request to get the data from this Collection.
https://api.themoviedb.org/3/collection/10?api_key=THE_KEY&language=en-US

If this data solves your problem then I suggest you download the list of all valid Collection IDs.
https://developer.themoviedb.org/docs/daily-id-exports

http://files.tmdb.org/p/exports/collection_ids_MM_DD_YYYY.json.gz  
Pay attention to the date format. MM_DD_YYYY.  

http://files.tmdb.org/p/exports/collection_ids_04_07_2024.json.gz
And make an API Request to obtain data from each of the Collections.

Hello, I've just made my own list of 500 movies. I think I've found how to do what I want to do. I'm gonna try to use my own list... Thanks for your help !