React is not even a framework, really. It is a great starting point for simpler sites. Honestly still one of the best solutions out there.
It is one of more clear examples of a framework:
- calls your code
- heavily affects how the code is written
- requires significant build configuration (could even be argued it is something more than a framework, but definitely not a library)
A library does not impose any configuration. I don’t see any required configuration with React. You can use it wherever you want in your code as an addition.
My piece on that: https://datacadamia.com/code/design/library_vs_framework