BBGW how is the initial WIFI setup with AP + client in parallel made?

Hi folks,

I would very appreciate if somebody could explain little quick how to setup a AP capable WIFI device like implemented on the BBGW to become an AP and at the same time is able to connect to an existing other WIFI network with an obviously different network address.

is this a hostapd feature?

What I am looking for is a starting point to learn about this setup. what packages are sw necessary for this?

Thanks a lot for any answer.

I apologize if this topic already exists and I just didn’t found it during my search. Links welcome.