Which is more popular Python or Java

Easy to learn Compared to the other top-notch framework to develop progressive web apps, React is much easier to understand . It is the primary reason for its gaining popularity among developers. The more harder a particular technology or framework is more time the development processes take to begin.

Based on what you decide, you can go forward with that, learning python gives you advantage of writing automated scripts, utility tools, backend etc. while learning react will help you in building VR & native apps as well.

Yes . React is mostly agnostic of what’s going on at the server-level. ie it does not care what backend you use. Not entirely true if you want to do server-side rendering of React components.