I have done a lot of search and find out that, without private api we can only get the SSID of the network your device is currently connected to, but I have tried Nest iOS App, this app could get the SSID list, why is that, did Nest use private api, why apple didn't reject Nest App.
I have figured out how Nest App achieve this: first, nest app should connect to the AP of Nest device (which contains a wifi module, and, of course it could scan the available wifi), and then the nest device will send the wifi list to nest app.