Does the iPhone's built-in cable have any protection?

If you are using the cable which came with your iPhone or an aftermarket MFi Certified Cable, it has all it needs to protect the iPhone. I truly don't know what protection features you are concerned about as I've never seen a rating for such and as long as you are using a good quality charging brick, there is nothing you need to be concerned about. It is not possible to overcharge an iPhone. Keep the cable in good condition (not frayed) and there is nothing to worry about.

Not sure what "protection features" you are looking for. Are you asking if the cable is shielded or something else? it is the iPhone itself that controls how it charges, so it cannot overcharge.

The cable really has no more or no less protection than other cables.

What builtin cable are you referring to?

Sure. Remember that the iPhone itself controls its charging. It cannot be overcharged.

Sure, it's safe.

You're welcome.