[aastronaut]

I'm honestly a bit confused by this statement, as "React env variables" can mean various things, depending on your setup.

Backend and frontend can be separate runtimes... The backend could be an API-only-server and an OS process with all permissions that come with it (reading env vars during execution as it pleases). The frontend, eg. a React SPA, could just be a bunch of built HTML/CSS/JS-files (type-checked, bundled, minified, etc.), served through a static file server (separate backend, nginx?) and interpreted through a browser engine on the client.

Are you asking for nginx to parse the served HTML/CSS/JS-files and replace unknown placeholders with env var values? If so you are prob asking for a SSR framework[0], meaning a backend that treats a React project similar to template files, and is able to inject env vars?

[0]: Next.js - https://nextjs.org/docs/app/building-your-application/config...