Well it's true that the iPhone Xs has one, I'm not sure about any other phone though. An e-sim is basically a sim that's built into a smartphone and unlike the nano SIM and micro SIM, it can't be removed. It's the latest advancement in phone technology which has seen all other components get smaller and screens get bigger.

The e-sim is about 15% the size of a nano-sim so it's actually pretty small. The e-sim is seemingly easier to use that normal s because provided your network provider supports it's use, activating the sim for a particular network is just as easy as downloading the app of your network provider and scanning a barcode and the sim will be automatically configured. This is great for people who travel alot because you can simply just switch network providers or carriers once you enter a new country.

It also helps keep your phone gear connected without the use of Bluetooth or a WiFi connection, to the best of my knowledge this holds true for iPhones and their watches.

The only downside to using these is that if you lose your phone or your phone is down and you want to put your SIM in someone else's phone and make a call then it won't be possible, otherwise it's a pretty cool idea if you ask me.

I hope this helps.