That entails analyzing all of their games and finding defenses and weaknesses. But also trying to find new novelties in the openings etc. And since the opponent will do the same, you yourself have to prepare defenses for lots of potential new openings 16 moves deep or so. It's an insane amount of studying.
Of course, that pays off for future games outside the match as well. But when you know your challenger will spend half a year on this once-in-a-lifetime opportunity to beat you, you probably have to do the same. So have to say no to everything else.
This of course gets unwieldy very fast. 16^5 is already over a million different games.
Though, like you said, the vast majority of those moves will be somewhat nonsensical though.
This is balanced by the fact that the sheer number of possible lines is huge. Too big for any human to "remember" them all.
It's also not enough to memorize "good" moves from an engine perspective. "Good" engine moves and "good" human moves are different things.
An engine might see a move as "good", because it calculated 50 moves down the line and found an advantage. This means that you'd have to play the next 49 moves in a very precise manner to reach this advantage. One way to think about it, is that there are multiple positions that are analyzed as "draw" by en engine, but 9 grandmasters out of 10 will see one side winning. That's because one side would need to play perfectly, while the other side has much higher tolerance for mistakes (engine assume perfect play on both sides).
TLDR: engine is fair game, during preparation. It's a big deal, it transformed high level chess, but it didn't break the game, and is unlikely to do so.
The ideal at top level is to get to move 20 or so with a slightly better position, essentially all of your time (because you've been playing memorized moves), and your opponent already having used most of their time.
They need hundreds such ideas for a match.
Remember for the best players in the world chess is their full time job. They spend 12 hours a day on the game. They are earning enough from tournaments to support life.
Yes. Their upcoming opponent's past games (including analyzing them for strengths and weaknesses), and engine lines mostly.
It sounded sort of like a side mind game that Carlsen plays on his opponents. It made it clear that Carlsen really studies his opponents and not just their past games.