"Modern” Web App Development

The last time I did web development from scratch for a frontend-heavy app (think unbundling Excel), the way of doing it was with a frontend in React/Angular/Vue/etc, and a backend REST API in anything. It seems like things have changed a lot, and I'm wondering how I could learn about that and evaluate the different approaches. Nowadays I hear people talking about their frontend framework (React, lots of Next, things like that) and that's bascially it. A few questions:

- How do you handle authn/authz? Do you use auth providers? Who are they, and what are the tradeoffs? Do you deploy yourself an auth API? Which one do people use?

- How do you handle persistance? With database providers, like Firebase/FaunaDB? How's testing?

- Do people just don't write backend anymore? I hear a lot about lambda functions, which seems great for one-off jobs (send an email, process an order), but I don't imagine that people write all of their backend with it. Are backend not a thing anymore?

- If that's the case, what are the tradeoffs compared to the "old" way of doing it, and the even older (server-side rendered HTML with lots of JS)? Do you gain development speed? Maybe you write more code specific to your "problem" and less plumbing that everyone needs? Would you consider this the best approach to develop a frontend-heavy app in 2022?

- How do you learn about all that? Read news often? Are there people making comprehensive reports/tech radars of what's used and how well it works? Are there good books on the subject? Blogs? Podcasts, videos?