There’s no need for something that complex.
Someone with access to a chess engine watches the game and inputs the moves into the engine as they’re played. If there’s a critical move (only 1 or very few of the options are winning/don’t throw the game) they send a simple signal to let him know. That can be enough to give you an advantage at that level. If you really want, you could send a number between 1 and 6 to represent which piece the engine prefers to move, but it’s likely not necessary.

That said, all the evidence he actually did anything like that is at best circumstantial (mostly statistical evidence supposedly showing how unlikely his performance was given his past performance and rating at the time, as well as known instances of past cheating by him - though the only confirmed ones were several years ago when he was still a kid and online rather than in person).