When it comes to Endgame, there are technically four game modes, but these first two are what most players think of when referring to endgame.
Shiyu Defense
Shiyu Defense are timed stages where your goal is to defeat an enemy or waves of enemies within a certain time limit. This is similar to Memory of Chaos from HSR or Spiral Abyss from Genshin. This game mode rotates every two weeks, allowing you to earn recurring rewards and tackle new content.
Shiyu Defense requires two teams to complete, meaning you'll need to invest into around 6 characters to tackle this content well.
Deadly Assault
Deadly Assault is the endgame mode that most consistent players care about. It's a boss rush where you have 3 minutes to deal as much damage to a boss as possible.
While still a timed mode, it provides a different gameplay experience from Shiyu Defense which is often a race. In comparison, Deadly Assault is more like a marathon.
Deadly Assault also resets every two weeks and requires 3 teams to complete.
Battle Tower
Battle Tower is a technically infinite tower that you can climb, defeating stages of enemies. After a certain point, the levels will simply repeat, so it's more a contest of endurance than anything.
You don't need to get too far to claim the content rewards, but the content is there to climb if you want. Unlike the above two modes, the Tower is untimed, but the enemies deal massive damage. You also don't get to heal, so avoiding damage completely is the focus of this content.
Hollow Zero
Hollow Zero is a "rogue-like" mode where you accumulate buffs to empower your team as you progress through a randomly generated map of levels.
While it's technically a rogue-like, the system itself is treated more like a weekly quest to complete for rewards rather than difficult content.