https://digitalgametechnology.com/what-is-chess960-or-fischer-random-chess

Basic Rules Chess960 aka Fisher Random Chess:

At the start of every Chess960 game, all white pawns are placed on the second row and all black pawns on the seventh row exactly as in a game of Classical Chess. The white chess pieces are placed on the first row and the black pieces on the eighth row in a mirror image of the white pieces, again just like in Classical Chess. The only difference with Classical Chess is that the position of the pieces on the first and eight rows is not in the traditional order but in a randomly determined order. When placing the pieces on the 1st and 8th row the following rules must always be followed:

Each player has one white squared bishop and one black squared bishop.
The king must always be placed between rooks to allow castling.

These restrictions result in a total of 960 possible different starting positions. This includes the standard Classical Chess starting position and 959 other starting positions. The actual rules to play Chess960 are the same as for standard Classical Chess. The only exception is the castling move as is explained below.

White and Black have identical and symmetrical starting positions. From their side of the board the player’s pieces are facing each other directly, symmetrically. For example, if the random shuffler places White's pieces in the following positions:
Ra1, Bb1, Kc1, Nd1, Be1, Nf1, Rg1, Qh1

Black’s pieces must be placed as follows:
Ra8, Bb8, Kc8, Nd8, Be8, Nf8, Rg8, Qh8.

Note that in Chess960 the same notation is used as in Classical Chess.

More details for rules:
http://hgm.nubati.net/rules/FRC.html

Top-Players:
https://www.freestyle-chess.com/freestyle-elo-table/
https://ratings.fide.com/top_lists.phtml?list=open
https://www.chess.com/de/players

Freestyle Chess Players Club (FCPC)
members need to have more than 2.725 points in https://www.chess.com/ratings

https://en.wikipedia.org/wiki/Freestyle_Chess_Grand_Slam_Tour

History:
10. - 17. November, 2024: Singapore
https://www.freestyle-chess.com/
https://www.freestyle-chess.com/news/a-cage-match-as-the-climax-magnus-carlsen-wins-the-freestyle-chess-summit-in-singapore/

7. - 14. February 2025: Weissenhaus, Germany
https://en.chessbase.com/post/freestyle-chess-grand-slam-tour-2025-kicks-off-with-gukesh-carlsen-and-anand
Winner: Vincent Keymer
https://www.freestyle-chess.com/2025-germany-weissenhaus/

8. - 15. April 2025: Paris, France
Winner: Magnus Carlsen
https://www.freestyle-chess.com/news/april-7-14-freestyle-chess-grand-slam-in-paris-press-release/

17. - 24 July 2025: Los Angeles, U.S.A
Winner: Levon Aronian
https://www.freestyle-chess.com/news/aronian-wins-first-freestyle-grand-slam-title-in-las-vegas-flash-report/

5. - 12. December 2025: Capetown (Kapstadt), South Africa
Winner: Levon Aronian
https://www.freestyle-chess.com/news/levon-aronian-wins-freestyle-chess-grand-slam-south-africa/

2026: probably Australia (City to be announced)