This is German Montes a Shopify theme developer from Mexico.
I have rich experience in Shopify theme development but I'm not sure with Shopify app development.
I'd like to learn about Shopify app development.
But I haven't ever developed Shopify app.
I would appreciate to hear from you If you give me some advice how to create Shopify app as custom app.
Check out the app concepts described here https://shopify.dev/concepts/apps to start!