A collection shows all the products that it contains.
What if you want to show sub collections?
There’s a page template included in your theme that can help with that.
For example, let’s say that you have a collection named “Star Wars” and you want to show sub collections for example a thumbnail for Empire related products, one for Droids, one for Jedi etc.
So in this example, you’d create a page named “Star Wars”. See the following guide if you need help with creating a page…
Shopify pages documentation
Once you’ve created the page, you’ll see a dropdown that allows to select a template. Choose the template named “page.sub-collections” and save your changes.
Now go the the Navigation section of Shopify admin and create a menu with the exact same name as the page. So in this example, you’d add a menu named “Star Wars” then add links to the menu. Those links will then appear when you view the “Star Wars” page in your storefront.
NOTE: if you don’t see the template in the page dropdown you could be using an old version of the theme or customizing a theme that isn’t published as your live theme.