iPhone Manual Carrier Selection
Your phone will not be able to access Sprint or Verizon networks, because it physically lacks the hardware to do so. The GSM radio in your phone cannot communicate with the CDMA signals that Sprint relies on.

Project Fi only works with T-Mobile and Sprint. From the list, we can rule out Verizon, Sprint and 310-830 (Sprint), so our only options are AT&amp;T and T-Mobile. You can try connecting to AT&amp;T if you like, it won't break anything - you'll be informed that you can't register to that network. Therefore, your only option here is T-Mobile (probably represented by Project Fi here). In your case, the Automatic toggle will always connect to T-Mobile because that's the only supported network.

Let's say you could connect to another network on that list, though. Let's say Project Fi supported both T-Mobile and AT&amp;T. Leaving the Automatic toggle on would pick whichever of those networks had a higher signal.

So, to answer your TLDR:


The automatic toggle connects to the network that a) provides the best signal strength, and b) that you're able to use.
It won't work. You'll be given an error, or will receive no signal at all.