To display the sub-collection list within a collection, follow these steps. It works with the metafield option, so create it by following the steps below.

Step 1 :

 Go to Store Admin > Settings > Metafields > Collections > Add definition

Follow the steps marked in the image below.

Step 2 : Now go to the collection page in store admin and scroll down to the metafield section.

Click on the “Sub Collections” field and select collections.

Step 3 : Now go to the sub collection section in theme customize and click on the “Dynamic Source” button and select “Sub Collections,” then save the page.