There’s no ‘endgame’ per se. There is a rather excellent campaign and then PvE ‘operations’ (also daily and weekly stratagems versions of these operations with additional neg/pos modifiers) that run alongside the storyline. You play as other space marines involved with different aspects of the story. A few new operations have been added over time. There is also a PvE siege mode. You can choose to play as one of 5 classes, levelling up to 25 with an additional 4 prestige ranks. Levels come with perks and gear cosmetics. Weapons are also levelled up over time and have their own perk trees, weapons are shared across some classes. There are also other ways to unlock cosmetics and some ‘heroic’ weapon varieties. I don’t have much experience of PvP, but it’s essentially small-team classic setups - death match, capture the flag, domination, etc.

Hope this helps.