This sounds like Sky Without Stars by Jessica Brody.
Three strangers, one shared destiny…
When the Last Days came, the planet of Laterre promised hope. A new life for a wealthy French family and their descendants. But five hundred years later, it’s now a place where an extravagant elite class reigns supreme; where the clouds hide the stars and the poor starve in the streets; where a rebel group, long thought dead, is resurfacing.
Whispers of revolution have begun—a revolution that hinges on three unlikely heroes…
Chatine is a street-savvy thief who will do anything to escape the brutal Regime, including spy on Marcellus, the grandson of the most powerful man on the planet.
Marcellus is an officer—and the son of an infamous traitor. In training to take command of the military, Marcellus begins to doubt the government he’s vowed to serve when his father dies and leaves behind a cryptic message that only one person can read: a girl named Alouette.
Alouette is living in an underground refuge, where she guards and protects the last surviving library on the planet. But a shocking murder will bring Alouette to the surface for the first time in twelve years…and plunge Laterre into chaos.
All three have a role to play in a dangerous game of revolution—and together they will shape the future of a planet.
This book includes Marcellus's grandfather, who is an antagonistic figure. And they have screens on their arms. From the description, this book appears to be nearly 700 pages long, fitting your description of "a lot of pages". There are also three books in this series.
