Gotify works with a server.
Right now Gotify is UnifiedPush’s primary distributor. There are public servers and more information available on the Gotify page
Firebase Cloud Message is Google’s push notification service. You won’t need anything else to use it.
NoProvider2Push needs a static IP to work, which is reachable with a VPN or public meshing network. It is mostly useful for development purposes and advanced users at this moment.