Asterisk looks for the [default] context whenever it has a call that does not have any other context associated.
If you have a ZAP trunk or a SIP trunk that has another context in it’s parameters it will use that one.
It’s the same thing for phone sets or anything else. They will begin in the context that you set for them when you create them. If you do not set a context, or the context is default, that’s where the call services will start.