About the SEO friendliness of React:
React sees itself as a UI library and not a framework. The reasoning is that it only offers basic UI features out of the box. Programming-model like I would consider it a framework.
Anyway, Next.js and GatsbyJS are seen as the frameworks for the library that React tries to be. They both come with pre-rendered HTML options that are more SEO-friendly. Next.js offers server-side rendering and GatsbyJS offers compile-time rendering.
I think this is also true for Vue.js, which in turn has projects like Nuxt.js and VuePress.
True, we could add here stuff like for example react-helmet as well.
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