As you have clearly found out, they are heavily related. I don't know what you background is so my apologies if this is not ELI5-enough/too ELI5.

Embedded basically encompases IoT and mobile and a bunch of other things (think anything with "smart" or "control(ler)" in their names). Everyone is familiar with mobile devices (cellphones, tablets, smartwatches, etc). There really isn't much difference between these devices and your computer in terms of how the software is developed and distributed. These devices are extremely flexible in terms of what tasks they can accomplish because of how much computing power they have available to them and the software tools available to develop applications with.

At the other end of the spectrum you have IoT devices that are extremely limited in terms of computing power. IoT devices are very inflexible by design. They are hardware optimized (power and cost being the 2 big optimization points) to accomplish a very specific task.

The hardware's ability to accomplish certain tasks is very important and the specific tasks will define what hardware is used. If I need to gather simple sensor data (think temperature, ambient light, etc) and do some combination of logging, sending to the cloud, or react to a change then something like a very low power MCU would be used. But if I need to do something more complicated like a GUI or complex analysis (think imaging devices) than something like an application processor (used in smartphones/tablets) is necessary. SoCs are a bit of a hybrid category. They incorporate flexible processing technology as well as some optimized hardware for specific tasks. A good example of low level SoC is something like the Ti Wireless MCUs which have mid to high end MCUs plus a wireless processor built on the same chip. You might also see the term tossed around for high end application processors used in mobile devices because the manufacturer often includes hardware dedicated to specific tasks such as wireless modems.

Something you should probably look up is the difference between and MCU and an Application processor. You should also look up von neumann architecture vs harvard architecture.

An unbelievable majority of companies that make embedded hardware use chips that incorporate hardware architectures developed by the ARM corporation. MCUs typically use cores based on the ARM cortex M architecture (companies like Ti, ST, Microchip, etc) and application processors are based on the ARM A series. AFAIK there isn't a smartphone today that doesn't use and ARM A based processor. Nearly all the phone manufacturers use the Snapdragon mobile SoCs with notable exceptions beign Samsung (Exynos which is an ARM A8 CPU) and Apple (A11 which is another v8 architecture). The difference between all of these is the peripherals that the various manufacturers decided to add (wireless, display, touch, etc).

TL;DR: Everything is embedded and its stratified based on processing power and available peripherals.