You can use WordPress as a REST API, which allows you to read/write data from WP in JSON format.
However, this won't be as performant as other database solutions, so you may want to consider alternatives unless there's a strong business reason to use WordPress.
I've built apps without WordPress, and also with WordPress as a back-end to meet a client's requirements, so I'd be happy to discuss the pros and cons with you if you'd like a second opinion.
You could use a plugin. AppPresser is a suite of WordPress plugins and theme that you install on your site, customize, then package into an app and submit to the app stores.
It has good reviews and it built by a team of very reputable and solid developers.