A view of a banners' assets can be found by navigating to 'Content > Assets' by using the dropdown menu placed at the top. An asset can be a downloadable file like a discount voucher .pdf file.
Assets for a GrandVision banner should only be added to its own folder. For example assets for MultiOpticas should only be placed within the MultiOpticas folder. As an editor you're free to create folders and assets as you see fit.