How to start creating an application?

Question

Mobile development

Hello, tell me how to start developing an application for the site? How is it done in what language? I heard that rest api is needed, but it's not really clear how to do it. I have a lot of php code, many parts of the code are maximally adapted for output to the Smarti template engine. There is authorization and a lot of confusing php code. How to deal with all this? Create a separate convenient script to somehow do api to it and connect a shared database to it? That is, the application and the script will be completely different programs, but work with the same database? Are there easier options?