Android and iOS don’t let mobile apps run continuously in the background. If an app is closed or in the background, it generally can’t talk to its own servers.
Instead, Google and Apple provide a service that allows the apps’ servers to push a message even if the app is closed.
If you take out the employer-side taxes and cost of benefits, maybe. A fair number of their employees must be software engineers, and that much compensation isn’t unreasonable for expert software engineers.
I have GPay and I frequently get notifications telling me to claim their reward points. Those notifications aren’t configurable separately from the payment notifications at the OS level. Super annoying.
I don’t know how easy it would be to migrate to your own local machine, but what you’re describing sounds like Desktop-as-a-Service. All of the major cloud providers offer this in some form.