Turn the server on in Settings and this machine's files open in a browser tab — the same app, on the other device. It will not start without an access token, the token is shown to you, and you can regenerate it whenever you like. Turn it off and it is gone.
Ships
One button. The listening address appears next to it.
The browser gets a prompt; the token is on the Settings page you started it from.
Stop, and the machine goes quiet again.
An add-on is not sold on its own — a bundle is, and holding the same add-on through two bundles is never charged twice. See the bundles.
Nothing behind a readiness slider — the Web tab has no feature gate, so holding the add-on is enough.
Download File Manager, then pick the bundle that grants it.
