I am in the process of creating an external mobile app and I am trying to create orders via the admin api. I have everything working except for actually taking the payments. I have heard/seen based on my research that I may need to create a Shopify app to grant my app privileges to actually create a new payment and process it through shopify. I am very fuzzy on what that process is to create the app. I am wondering if there are any apps on the store that anyone knows of that has that functionality? Or if there isn't, I would love to get a little more clarity on what my app's redirect url and endpoint need to do for building the shopify app? Thanks!
Depending on your needs, consider looking at creating transaction records on an order through the Admin API (e.g. these API docs), or the Sales Channel API docs to create and update checkouts/process payments etc.
As far as tutorials go for an app that you would like to process payments on, check out these ones to get started: