Google crawls SPAs by letting their crawlers simulate the "server side rendering". Server side rendering has nothing to do with crawling private content. Those are off limits for Google. What you're thinking of is called "Deep Web" <a href='https://en.wikipedia.org/wiki/Deep_web'>https://en.wikipedia.org/wiki/Deep_web</a> and by definition those are not crawled by Google or any other search engines.
Google crawls SPAs by letting their crawlers simulate the "server side rendering". Server side rendering has nothing to do with crawling private content. Those are off limits for Google. What you're thinking of is called "Deep Web" <a href='https://en.wikipedia.org/wiki/Deep_web'>https://en.wikipedia.org/wiki/Deep_web</a> and by definition those are not crawled by Google or any other search engines.