When working in a restaurant, how does new wait staff learn the policies if they don't start out as host?

some restaurants don't hire the new wait staff as a host they just put them right up to server, but isn't the hostess the position that knows all the policies on guest services, restaurants rules and procedures etc ?

The host or hostess are required to know all the policies in order to perform their job function. You work up to that position from being waitstaff.

There is no set rules and at a small restaurant they can change on a whim. The boss will explain it. Serving food is an inborn knack. You either have it or you don't.