Acting on behalf of a user from web server

[penx]

Do any of the Firebase SDKs allow me to make calls to Firestore from a web server (eg node, nextjs, remix) on a users behalf, respecting security rules for the authenticated user and supporting multiple simultaneous users?

I read this:

If you are developing a Web or Node.js application that accesses Cloud Firestore on behalf of end users, use the firebase Client SDK.

Which suggests this should be possible, however my understanding is that:

firebase-js-sdk can only have one user authenticated at a time so would not be safe if handling async calls from multiple users at once

firebase-admin can be run as a specific user, but this can only be done during initializeAdminApp and again would affect all requests that are currently using the firebase-admin import

Is that correct? Are there any other ways to act on behalf of the authenticated user from a web app?

The only thing I can think of would be the REST API…
https://firebase.google.com/docs/firestore/use-rest-api