I'm trying to code the user authentication feature for my app using API. I'm confused between WordPress REST API and WordPress JSON API? Based on the Q&A here, it says I need WordPress JSON API. If my WP already has REST API, do I still need this JSON API?
Yes, they appear to be different. The Wordpress REST API appears to be built into, and come with a Wordpress Installation. I tested on my site, and was able to access the api for posts via a browser GET to /wp-json/wp/v2/posts. This is just an example. I would imagine the other option you mentioned is a plugin that does something similar but the format of the requests look entirely different. I would stick with the first solution that comes out of the box.