Is there a place I can find a "current state of the art" on this stuff? Once upon a time a quick-start option would have been. For instance, I may have done Rails+React+Babel+Webpack to get a quick backend JSON-API and a front-end web-app that consumes it.

Not for something that "scales to 100 teams", just something for me to spin an app up in quickly. i.e. 0 to 1 is the only thing that matters.

Next.js is pretty much that. Not much configuration needed, and it wires up all the tools for you behind the scenes. If a new tool comes out that's worth it (like potentially Turbopack, although that's by the same company), they'll do the migration for you.

Rails also has a whole model/database thing going on.

Next.js is just front end focused and has always left it up to the user to decide about data persistence.

So to answer the original question, an equivalent would be Next+Database, and there is obvious stand-out answer for what Database should be. I often get stuck deciding what that Something should be when trying to go from 0 to 1.

Okay, great, I'll just use Next.js with PostgreSQL.

Getting Next.js up to Rails feature-wise is using something that already wires up the niceties like Auth, ORM, Tailwind styles, and Typescript. Create-t3-app is a huge contender here: https://init.tips/ and https://github.com/t3-oss/create-t3-app

A DB is as simple as pressing a button on Planetscale or Railway, copying the secret strings to your .env file, and 'npx prisma db push'.