I don't think it's merely mental albeit it seems like even nervous Carlsen is cooler than his very focused opponents (see game 3 vs Fabiano where caruana had a completely winning position after carlsens blunder).

Carlsen has spent the core of his career mastering two aspects historically underlooked aspects of the game.

The first is the endgame, and there isn't much to say there. He's by far the best end game player by far and it's not even close.

The second are drawish locked positions where most GMs can't but see a draw. Carlsen realized that in order for it to be a draw his opponents still have to play perfect and he focused a lot on accumulating small but convincing advantages in those kind of games.

Another thing that should not be overlook: mental strength, like you point out.

They had a heart rate monitor at one of the freestyle events which physically affirms what you're saying here. Carlsen's heart rate was barely above resting while his opponents were invariably like they were running a marathon. Even when he was losing, he remained calmer than when his opponents were in normal positions.

I think that should be a normal part of chess competition. It provides some really interesting metadata for spectators. To some degree it also emphasizes the importance of something people don't normally associate with chess - physical conditioning. When your heart is pounding for hours and the cortisol flowing, you literally get physically exhausted.

IMHO a huge aspect of Carlsen mental strength isn't just the focused, at-the-game part, but we just see him enjoying Chess in many angles: not only he plays all styles, he streams relaxed, he plays Lichess and Chess.com; Chess is not only his job and passion, but it seems that he's also able to relax while engaging with it.

The only top-athlete that I see do the same is Max Verstappen, who is know to play competitive racing-sims online even hours before a real F1 race.

Carlsen has always had a tenacity that allows him to come back from positions other players would give up on thinking to conserve effort to fight another day. Mental strength and stamina to stay in the fight has always been something that made him who he is.

You also can't underestimate physical stamina. Kasparov in his 5-3 result against Karpov in 1984-85 was eventually halted due to Karpov's exhaustion and losing 18kg over the match period.