G: TL;DR: No, plugging your iPhone 12 into your laptop via USB does not automatically share its cellular connection. But you can enable USB tethering manually.

How to Use iPhone Cellular on Laptop via USB:

1. Connect iPhone to Laptop via USB.
2. On iPhone:
   • Go to Settings > Personal Hotspot.
   • Toggle "Allow Others to Join" (if needed).
3. On Laptop:
   • It should detect the iPhone as a network adapter and start using its internet, assuming tethering is allowed by your carrier.

Note: Your carrier must allow tethering. If it doesn't, the option may be disabled or cause an error.

Let me know if you're using Windows or macOS and I can walk you through OS-specific steps.