When interacting with a web app using FIDO on a desktop browser, the browser allows users to choose which athenticator to use. These typically include built-in hardware devices, usb keys, or mobile phones.
How does the browser determine the list of devices to show to the user, and can a web extension be added to that list?
Passkey providers should plug into the operating system (using native code), which allows passkeys to be available across all browsers and apps, instead of siloed to a single browser.
With that being said, there are some platforms who do not yet have native platform APIs, so the only option may be to run as a browser extension. In this model, the browser extension intercepts the request, acting as the WebAuthn client instead of the browser/platform. The browser extension is then in full control of the request, and can either service it, or punt it back down to the original WebAuthn client.