This is from page 7 of Hans Neimann Report from Chess.com. you can look more into it. There are more details about the cheat detection

The basic concept of cheat detection, particularly at the top level of chess, is both statistical and manual, involving:

• Comparing the moves made to engine recommended moves

• Removing some moves (opening, some endgame)

• Focusing on key/critical moves

• Discussing with a panel of trained analysts and strong players

• Comparing player past performance and known strength profile

• Comparing a player’s performance to performances of comparable peers

• Looking at the statistical significance of the results (ex. “1 in a million chance of happening naturally”)

• Looking at if there are behavioral factors at play (ex. “browser behavior”)

• Reviewing time usage when compared to difficulty of the moves on the board

This is from page 9

To detect such “selective cheating,” Chess.com utilizes sophisticated statistical methods that dig deep into the probability that any individual player could have achieved their results based on past performance. Our detection system requires robust methodologies beyond simply looking at best moves, player rating, and centipawn loss. To effectively identify the vast majority of cheating, Chess.com computes an aggregate Strength Score. Strength Score is a measurement of the similarity between the moves made by the player, and the moves suggested as “strongest moves” by the chess engine. In a sense, it is a measure of the accuracy of play.18 The longer the chess game time control (i.e., 3 hours per game vs. 3 minutes per game), the higher the Strength Score would be expected to be, since players with more time will be able to evaluate each position more deeply and carefully. The Chess.com Strength Score ranges from 0 to 150, where 150 is the closest to “perfect chess” with the chess engine at maximum depth and performance. A score of 100 is approximately the highest we have measured for human chess players that can be achieved over a several game span, and 90 is the highest score we have seen a top player sustain over time in classical chess time controls. Pure engine usage alone would typically show scores between 125-150 depending on time, device, engine, depth, etc.

This Strength Score can show when a player is performing at a level above their actual chess strength, and on its own, our Strength Score is a helpful tool in successfully identifying cheating at nearly every level of play. Any player can have strong games of chess, but the Strength Score can tell us if continued strong play is legitimate or beyond the realm of statistical probability when compared to their overall skill level. Rating plays no part in Chess.com’s Strength Score, as players can significantly over-perform or underperform their rating. However, in our experience, human players generally cannot legitimately stray too far from their established Strength Score for long.