Hi Halley,
Most Shopify apps are created by third-party developers, just like the apps on your smart phone. Some apps are free, while others are not. When you do purchase an app, you're paying for the functionality that the app provides and the work of the developer, not for hosting. You can view all of the free Shopify apps by visiting this link: https://apps.shopify.com/?filter=free.
If you're looking for a free app to edit metafields, MetaField Editor is a well reviewed app that you could try: https://apps.shopify.com/metafields-editor
Your theme only determines the look and feel of your customer-facing website and not your Shopify admin. So, unfortunately adding a JavaScript file to your theme wouldn't allow you to edit metafields. The only way to add additional functionality to your admin is through apps.
Hopefully that helps!