What is going on "under the boot"?

For my own knowledge I would like to know exactly how my NAS (that is on a Local Network in a home, behind a router that is on a dynamic/virtual IP) can serve contents directly to an external device. I understand that the NAS is continuously announcing itself to the WD server so when from the other part of the world with my mobile phone I connect to WD server, my phone receives the exact address to connect to my NAS that is in my house.
What I don’t understand exactly is… what info does this address contain?
I believe it will be composed by the real IP my ISP uses for me on the internet (no matter if shared with other users) plus some other data that are specific to my MyCloud NAS. possibly this is a PORT on the ISP router? So once the ISP router receives a request on that specific port it knows that this needs to be redressed to my home router. And how does it tell my router that the request is exactly for the NAS ( I might have other devices connected). Maybe my router too is instructed?

So… basically… let’s say I want to contact my NAS from my phone directly (outside from the WD MyClou application) what address do I have to use?