It's quite possible to use react with nothing else: no build tools, no transpilers, no fancy JSX, so in terms of tooling complexity, it can be as simple as you want or as complex as you want.

Regarding the complexity of the underlying system, I think this is not a concern of the typical frontend developer. It has nothing to do with JavaScript or the web. If you are writing a "simple" C program on Linux, would you be concerned about the complexity of the underlying C library (yes glibc is rather big and complex) or the Linux kernel? You won't. This is the point of abstractions. If you want, you don't have to learn about any of these; treat the implementation as a black box and start writing actual frontend code.