@Abdelhak , well, I'm not sure about the first question, but about second is yes, because the switches will "download" the configurations applied on the dashbord.
About the first one, do you want to have two separated physical networks but just have one Wan internet connection? The switches don't need to have a public IP, they can use a private IP, they just need internet access.
The internet connection is not yet activated by the provider so if we start racking the switches and connect the access switches with MS425 switch , the network will be ready to use or do we have to wait until we create the network on the cisco meraki dashboard ?
when we add a switch to dashbord , it have to have be connected perminatly to internet or just a moment to upgrade to firmware and download the configuration ?
The switch must connect to the internet at least once so that it can download the settings and update the firmware after that, it works even without an internet connection, but to make any future changes it must be online in the dashboard.