They Both Die at the End by Adam Silvera

The narrative follows Mateo Torrez and Rufus Emeterio, two teenagers who receive calls from Death-Cast informing them that they will die within the next 24 hours. Seeking companionship on their final day, they connect through an app called Last Friend. As they embark on a journey across the city, they form a deep bond, confronting their fears and embracing the moments they have left.