Any post or article where the author is like "Let's build a React" may emphasize vanilla JS. For example,
HEX: a No-Framework Approach to Building Modern Web Apps (2017)
https://medium.com/@metapgmr/hex-a-no-framework-approach-to-...
Manning has Build a Web Framework (From Scratch) in its Manning Early Access Program (MEAP):
https://www.manning.com/books/build-a-frontend-web-framework...
Web Components are a neat way to have <custom-tag /> stuff controlled by CSS. But you can forego that and use (BEM-named) values in class attributes.
Even just `document.querySelector` and `.querySelectorAll` gives some idea of vanilla JS. There's also (tongue-in-cheek)
http://vanilla-js.com/
This book for writing Javascript may still be relevant:
Reliable Javascript: How to Code Safely in the World's Most Dangerous Language (2015)
Hmm, code samples are no longer easily available. I would love to share it if Wrox Press (formerly) were cool with it.
And then there's places like Mozilla Developer Network (MDN):
https://developer.mozilla.org/en-US/
You mentioned backend. With just Node.js, you're technically running vanilla JS there too.