How do Grandmasters think in an unknown position and make the best move?
When a GM, or even lesser strong players reach a position that is totally unfamiliar, they have to break it down into components. They evaluate the following for BOTH sides. In general, a lot of this is done subconsciously by strong players.
- Material, and what pieces are better. Sometimes a well-placed knight can be better than a rook, for example.
- Can any of the pieces be shut out of play?
- King safety.
- Are there any direct threats?
- Pawn structure and any corresponding weaknesses. (like doubled pawns, but not all doubled pawns are weak or bad as sometimes they provide great protection...more in the endgame can they really be a big problem, but you have to judge this carefully.)
- How many pawn islands does each side have? (less is usually better)
- Where are the pawn majorities? (Usually a q-side pawn majority is easier to mobilize)
- Passed pawns.
- Who has more space?
- Who better controls the center?
- Are there open lines, and who can control them?
- Are there half-open files that you can put pressure down?
- Strong outposts like being able to put a knight on d6, or a bishop on h6. This can disrupt communication between the two sides of the board, and make defense moving from one side to the other very difficult.
- Any colored square weaknesses, like being weak on the light or dark squares.
- Any lead in development for either side.
- Which trades are good for me, and bad for him? (exchanges are one of the least written about subjects in chess, but they are one of the most important factors in chess as you get stronger. Too bad it has not been translated, but there is a book in Russian called "The Encyclopedia of Exchanges".)
- Related to number 16, and that is who has the better endgame if certain trades happen?
Finally, based on these factors, players weigh them based on experience and importance, and then they come up with a list of candidate moves, and they calculate likely continuations.
I might have missed a point, but that is a pretty complete list.
Depends a lot from player to player and also position to position I think.
The fundamental is that their intuition (built up from studying and playing and solving a lot) will suggest a few moves (or occasionally only one) and they will calculate those moves (candidate moves) and pick whichever they think is best based on calculation and evaluation.
They might find more candidate moves suggest themselves whilst they are calculating the first ones.
GMs think differently. Most players would use pattern recognition, intuition, and calculation of variations for candidate moves.
Some GMs will often think past the board and consider the opponent.
They also do other things that are beyond my pay grade. I read about GM thinking once and it convinced me to give up that goal. I just enjoyed playing as I could not think like they did in what. To me, it was somewhat contradictory and illogical in ways.