Switch carriers manually on the 11 iPhone

What exactly do you mean by "manually"?

If the iPhone is unlocked, then simply inserting a SIM card from a different carrier or setting up an eSim from a different carrier would be all it takes to switch carriers.

If the iPhone is locked to a specific carrier, then only the carrier it is locked to can unlock it for use with other carriers.

click here ➜ How to unlock your iPhone for use with a different carrier - Apple Support

And for completeness, here is how to use dual SIM/eSIM on an iPhone 11:

… Use Dual SIM on iPhone - Apple Support