What is the difference between Firebase API key (a.k.a Server Key) and the Web API Key?

From my answer here:

• current key - nothing is explicitly stated anywhere in the docs where it is used, however, among the 3 services where the google-services.json was originally used for before Firebase (Google Sign-in, Analytics, GCM), I'm guessing it would be between Google Sign-in and Analytics. As also mentioned by @DiegoGiorgini here:

The api_key value in google-services.json is not used by FCM. (it is used by other Firebase libraries)

So it can be for a separate Google non-Firebase or Firebase service.

• Web API Key - this one I haven't had a chance to use before. However, I've seen other posts wherein this specific API key is used often on Web apps related to Firebase (possibly also for Auth purposes?), but I'm not entirely sure.

• Server Key - as per the docs:

A server key that authorizes your app server for access to Google services, including sending messages via Firebase Cloud Messaging.