Add to cart in Shopify with API

I just started using Shopify and it’s APIs but I’m unable to complete my checkout process.
I’m using PHP as a backend and creating a react-native app. how can I achieve this?
I checked the Shopify-PHP-SDK, REST Admin API but didn’t find anything helpful.

This is the first app that I’ve built using the Shopify API, so I’d appreciate any guidance that I can get. I looked into signing up with a partner application and using the OAuth authentication, but that seems like overkill for what I need to do since I don’t want to use the Shopify storefront and I don’t plan on offering my app in the Shopify App Store.