FOSS basically goes against the concept of private property of software and embraced common ownership of software. Without private property, there is no capitalism. I wouldn't call FOSS communism or socialism, but there are elements in it.

You wouldn’t have react.js without capitalism.

Ok, and what's your point? If you read Marx, one essential point he claims is that without capitalism, there cannot be socialism.

They could have used different library for js. one made totally by volunteers, but they haven’t. Why?

Probably because they saw no use in reinventing the wheel? And why should they?

It's as if you told a revolutionary during the French revolution "You used weapons that you looted from the Bastille, weapons that were produced by the king.". What exactly would be the argument here?