The Underdeep friend points login bonus is the reason the friend list matters mechanically: every friend who has logged in that day contributes 5 loop tickets, ten friends give the maximum of 50, and on top of the 5 tickets you get for logging in yourself that makes 55 tickets a day.
How you make a friend
Your friend ID is a 16-character hex code. You find your own in the Friends list by selecting yourself and pressing the copy-ID button, and players post their codes so others can add them.
These are in-game friends, not Steam friends. You make one when you meet another player in the dungeon: you can send the request during the encounter itself, or later by clicking their name in the run summary. Players who go this way report gathering ten friends within a couple of days, and the other players guide explains what the encounters actually involve.
The login bonus is the reason friends matter
Every friend who has logged in that day contributes 5 loop tickets, and ten friends give the maximum of 50. The tickets themselves are what run your route while you are away, and the daily numbers are in the loop ticket guide.
Friend points are a different currency
Friend points are the separate gift currency that other players send you, and they are what the Shop spends on skill advances. That is why the friend-point price of shop purchases matters so much, and why players keep asking for it to be lowered - the currency is slow to earn because it comes from other players' gifts rather than from your own runs. What the Shop sells with it is in the shop advances guide.
Underdeep friend points and login bonus: keeping the list rotating
Inactive friends stop contributing, so the list needs rotating: swap out friends who have not logged on for a while for new ones you meet in the dungeon. The full loop is to meet players in the dungeon, add them as friends, collect the login-bonus tickets, farm the dungeon for Orbs and Coin, and spend Coin on avatar unlocks that raise your stats.