I have to import 1000+ products and I am making a csv file with all the necessary columns and I need to import more than one collection for one product.
How can I do that?
The import file contains categories/ subcategories and sub-subcategories so I have to create a collection for every level and assign the product to all.
Have you considered using automated collections? https://help.shopify.com/en/manual/products/collections/automated-collections/auto-create
You could just import the categories, subcategories etc as tags for each product and they would then assign to collections based on how you automate them.