customizable mobile app use use the app intergraded in Shopify website througth Shopofy API

[Hayden1231]

If I already have Shopify website, I have intergrate some app, then I want to create a fully customizable mobile app, I can use the app intergraded in Shopify website througth Shopofy API, if Yes, which one API I should use. Thank You very Much!