>So, the UNA API too is written in PHP, right?

Right

>Messenger uses the Node.js framework

Yes, with PHP + Javascript with the Node.js+Tailwind for desing part. The JOT server has been developed with Node.js too.

>And my question #7 was about the language and framework used for UNA mobile app and NEO.

UNA mobile has description about it here:

https://github.com/unacms/una-mobile-app/blob/master/README.md

React Native + Node.js too.