An embedded-SIM (eSIM ) or embedded
universal integrated circuit card ( eUICC ) is a form of programmable SIM card that is embedded directly into a device.
In machine to machine (M2M) applications where there is no requirement to change the SIM card, this avoids the requirement for a connector, improving reliability and security. An eSIM can be provisioned remotely ; end-users can add or remove operators without the need to physically swap a SIM from the device. [1]
eSIM is a global specification by the GSMA which enables remote SIM provisioning of any mobile device, and GSMA defines eSIM as the SIM for the next generation of connected consumer device, and networking solution using eSIM technology can be widely applicable to various Internet of things (IoT) scenarios, including connected cars (smart rearview mirrors , on-board diagnostics (OBD), vehicle
hotspots ), artificial intelligence translators, MiFi devices, smart earphones , smart metering , car trackers , DTU, bike-sharing , advertising players,
video surveillance devices, etc.
The surface mount format provides the same electrical interface as the full size, 2FF, 3FF and 4FF SIM cards, but is soldered to a circuit board as part of the manufacturing process. The eSIM format is commonly designated as MFF2 .

Since 2010, the GSMA had been discussing the possibilities of a software-based SIM card . [3]
While Motorola noted that eUICC is geared at industrial devices, Apple "disagreed that there is any statement forbidding the use of an embedded UICC in a consumer product."
A first version of the standard was published in March 2016, followed by a second version in November 2016.
In 2016, the Samsung Gear S2 smart watch was the first device to implement an eSIM.
In 2017, during Mobile World Congress, Qualcomm introduced a technical solution, with a live demonstration, within its Snapdragon hardware chip associated with related software.
The information on an eSIM is rewritable, meaning you can decide to change your operator with a simple phone call. They're really easy to add to a data plan – connecting devices with eSIMs to a mobile account can be done in minutes. And, as we mentioned, it was 

eSIM is backed by the GSMA, the association of mobile networks and that organisation has defined the standard for eSIM worldwide. 

What devices support eSIM?

Apple has turned to eSIM for the iPad Pro, Apple Watch Series 3 and Watch Series 4 and Series 5 as well as dual SIM support for the iPhone XS and XS Max and iPhone XR as well as the new iPhone 11 and iPhone 11 Pro and Pro Max. Could next year's iPhone 12 move completely to eSIM? 

Google's Pixel 2 also supported eSIM but it was originally only used in the US for Google's Google Fi. The Pixel 3, Pixel 3 XL and new Pixel 4 and Pixel 4 XL do as well.

How to activate Google Fi on your iPhone using an eSIM

As we mentioned, eSIM support has also come to the Moto Razr flip phone . 

There's also eSIM support inside Windows 10 and some devices with cellular modems - such as Qualcomm Snapdragon 850-powered PCs - can use eSIM as an alternative to sticking a traditional microSIM in the slot. 

What networks support eSIM?

eSIM is available from selected carriers. You'll either need to have a carrier's app or a QR Code you can scan. Again, the carrier will need to support eSIM.

In the UK, EE, O2 and Vodafone support eSIM, although Vodafone is currently only supporting it for Apple Watch Series 3, 4 or 5 Cellular. 

O2 recently launched eSIM on compatible Apple and Pixel phones. O2 says that customers need only pop into your local store or call customer services to get an eSIM pack or download an eSIM. 

Let's look at EE's SIM pack. With EE you can get a traditional-looking SIM pack from a store like the one pictured here. But there's no SIM in it; instead, you get instructions and a QR code enabling your device to pick up the details. Each eSIM pack comes with its own number just like a traditional SIM pack. 

POCKET-LINT

In the US, AT&T, T-Mobile USA and Verizon Wireless support eSIM.

Away from the traditional carriers, "global mobile network" Truphone has started selling eSIM data plans. These are able to be bought via the MyTruphone app. Truphone's international plans work across 80 countries including in Europe, the Americas and Australasia. 

The app was originally available only on iOS but it is now available on Android for use with the Pixel Pixel 3, 3a and 4 (plus their XL versions). Truphone is currently offering 100MB of data completely free to new customers to try out the app.

TRUPHONE

eSIM is great for regular travellers

Theoretically, eSIM should mean that you could go to another country and simply add a roaming eSIM to your handset while retaining access to your main "home" number. That's one of the disadvantages currently because, if you change SIMs abroad, for example, you can't access your own number.

"eSIMs have the potential to eliminate roaming charges abroad", says Truphone's Steve Alder. "It also enables people to switch operators quickly to get connected if they’re in an area with no signal, frees up space for new features or additional battery life and could lower the risk of device theft.

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"As consumers and operators begin to see the benefits, mass adoption of eSIMs will be inevitable."

What does eSIM mean for devices?

One of the advantages it offers phone makers is that we should get smaller devices because there's no need to accommodate a SIM card or the tray that holds it. There's also no need for networks to manufacture or distribute lots of SIM cards. 

eSIMs will also be great for other devices like laptops and tablets, where seamless connectivity will become the norm. 

Vodafone says that eSIMs will enable more connected devices simply because eSIMs don't require so much room inside a device, enabling fitness trackers or even glasses to have stand-alone 4G or 5G connectivity in a way they just weren't able to before.

How will eSIM work in practice? 

We spoke to EE to ask how it will work with eSIM. The network told us that if you have a physical and eSIM provisioned and are connected to two separate networks, your iPhone will display both networks on the screen at the same time.  

If the handset is in standby and both the SIM and eSIM are provisioned, customers will be able to receive calls and texts on both numbers. As you can see in the instructions below, you can then choose a “default” line that you make calls on, use SMS and that iMessage and FaceTime use. The other line is just for SMS and voice. 

Alternatively, you can choose to Use Secondary for cellular data only - useful if you're abroad and using a local data eSIM. 

You can store more than one eSIM in your iPhone, but you can use only one at a time.

You can switch eSIMs by tapping Settings > Cellular > Cellular Plans and tapping the plan you want to use. If you are in the UK it's Mobile Data. Then tap Turn On This Line.

How to use eSIM with iPhone XR, XS and XS Max

If you have a QR code:

1. Go to Settings > Cellular.
2. Tap Add Cellular Plan.
3. Use your iPhone to scan the QR code that your carrier provided - you may be asked to enter an activation code.

Here's what the inside of the EE eSIM pack looks like (obviously we have blurred out the QR code )

cc: lalasticlala
mynd44

Funnily enough, if sb hacks into your eSim, it's gone. Some Techgurus say a user can be tracked easily too.

Ok

To all of the above, I would like to add about the safety of using the built-in SIM card. Unlike its physical counterpart, it is less susceptible to hacking. Source: https://esim-man.com/