How can I add products to the collection I created?

You should go to the 'Brands' page on the home page and go to the brand detail page of the product you want to add to your collection. Here, you can view all the products of the brand from the 'All' button next to the 'Products' heading, and add the product you want to your existing collection by pressing and holding the save icon and clicking the '+' button in the compnent that will open.