Hey there, @Ovidiu
Thanks for reaching out today. I definitely understand the importance of a terrific organization setup when dealing with such a large catalog like yours. It will be crucial in order for customers to easily find what they are looking for. With that said, just so I know we are on the same page, do you have any examples that you could share that resembles what you are looking for?
Just to confirm, it sounds like you want a collection page to take a customer to a subset of collections? For example, let's say you are selling fashion accessories and one of your collections is "Sunglasses". When someone clicks on "Sunglasses" on it would take them to a list of several collections that are for a variety of different frames of sunglasses such as "Aviators", "Round sunglasses", "square sunglasses", etc. Does that sound about right? If this is what you are looking to accomplish, we actually have a step-by-step guide here that you can follow to get this set up on your store.
If there is anything else I can help you with, please let me know.
All my products have categories/collections attached. I want new created Collections to be able to have products of other already existing collections because I don't want to search 40,000 products to add the new added Collections.