Though I don't know about Cell phone and smartphone transmissions completely I can share how things are done in a simple manner

 First of all, I assume (hopefully I'm right about this) that cell phone transmissions worldwide are similar to how the internet works simply because I could see the internet could make a good foundation for cell phone transmissions and because phone calls also travel over the internet worldwide, especially Signal and What's App and any other applications like this.

First of all to have cell phones work you need to move from Analog (old land lines) to digital transmissions which became possible more with the digitizing first of music into CDs and MP3s.

After music became more digitized into zeros and 1s like binary programming then phones could then more easily work that were also digital sound transmitters.

Then in order to have cell phones you need to combine digital sounds with Global Positioning Satellites which were invented so military jets in the U.S. stopped crashing into each other because of how fast magnetic north is moving now. So, the main reason GPS was invented was a military reason. But, with GPS then you could have mobile phones too in the form of Cell phones and then Smartphones when Steven Jobs invented the first real smartphone that we are using today in the form of an Iphone which is the simplest system to use but also the least secure safety wise too of all the smartphones.

So, with GPS and digital sounds capable of traveling all over the world both through Microwave towers and Satellite towers and digital sounds moving over the Internet in a two way or one way fashion we created the cell phones and smartphones of today starting around 2007 with the first Iphone Smartphone. Now we also have Androids and other systems as well. But, when I asked Google AI it said that mostly now the IOS of Apple and the Android system have most of the market share as of now worldwide.

However, if you are going to give a smartphone to a child or a newbie the simplest phone to use is always an Iphone even if it is less secure than an Android. However, that really isn't saying much for a variety of reasons because of how insecure all cell phone operating systems are (vulnerable to hacking of various kinds as well as listening.

This is why so many people use Signal and What's app to communicate especially around the world because they have end to end encryption so it is very difficult (if not impossible) for most people to listen into Signal or What's App voice or text communications.

So, when you combine digital sounds 2 way with digital music with GPS signals then the packets of information (sounds or texts) can travel all over the world either through microwave towers or satellites or a combination or through the Internet which is often through satellites too partly as well as internet cables of various kinds from Fiber Optics to others.