SIP "trunking" usage basics and vocabulary

When using a provider, how does the call routing work? It’s no different from dialing an internal number, is that correct?

I’ve been a bit narrowly focused on dialplans so that I’m not sure of the vocabulary involved here. The book I’ve been reading, the definitive asterisk guide, focuses on inbound calls. For outbound calls, how is a specific number associated with, or specified?

The provider uses the user part of the URI in the request line, whereas the phone might not (although it might also use it to select a particular line). Asterisk acts as a provider when you make a call from a phone.

Ok, and for the user part of the URI, the SIP “trunking” provider company will map, or route, each user to a specific phone number for caller id? Calls in/out to a specific number map to a specific user, and vice-versa? This is what SIP “trunking” providers offer?

For the mapping, or routing, multiple users can share the same outbound number, 1800, and then any inbound calls are routed to a specific number? This is configured through the SIP trunking provider, probably with a web interface?