Thanks Bhushan Jawle, spot on!
1. ...or recommended for only single page web apps
React really shines when the data in the app changes often as a result of user actions, server events, passage of time... So building a static blog with React is probably overkill. But not necessarily. The comments section of a blog post is perfectly suitable React app. In fact the first ever React app is the section for comments/likes/etc underneath Facebook newsfeed posts. React was designed from day one to fit into existing apps/pages. You can replace a small section of a bigger page. You don't need to rebuild the whole app (from
<html> to
</html>) from scratch
2. Is react used to design web application which are accessible through mobile devices?
You can render a React app to DOM. Then style it with CSS however you want - for for big screens, small screens... up to you. React is agnostic of all this.
Then there's also
React Native where you render a React app natively (no webviews) to Android or iPhone. Or
Windows. Or
Ubuntu.