Files, Collections are different location/functionalities i.e. Files will take you to /content/dam where you can upload assets and Collections will take you to /content/dam/collections where you create collection of similar type of assets.
Yes, you can add more items/links in Assets and for that you have to do overlay navigation ie. /libs/cq/core/content/nav/assets.
Document : experienceleague.adobe.com/docs/experience-manager-65/developing/platform/sling-resource-merger.html
FYI : As a good practice, please try to avoid customization.
