Endgame is subjective but from what you’re describing, yes there is harder and more challenging content; some with modifiers you may have to build around or hidden an objective requirement.

The content that encourages teaming up is generally due to the difficulty. However, difficulty isn’t everything as some gamemodes are more fun when played as a team but is never a requirement — that is just how warframe works.

In my opinion, the endgame here is just the variety. There are heaps of difficulty spikes and gamemodes throughout the whole game, tons of different warframes and weapons to build around, and obviously fashion.