1. Placing minimum bets when odds are in the casino's favor. 2. Placing large bets when odds are in the player's favor.
This makes the betting patterns of solo counters very different and easily identifiable from average joes.
Some players do try to add some variance and make intentional non-optimal plays to avoid detection. But every non-optimal play costs money and doing it too frequently eats up any potential profits.
Adding more decks doesn't fully prevent advantage play. More decks aren't harder to count. Adding more decks lowers the variance of the odds distribution through a shoe.
In a one-deck shoe the odds vary in a "spiky" way. It's more likely to be highly favored for the casino or highly favored for the player.
An eight-deck shoe the odds vary in a far smoother way. Most of the time they'll be slightly in favor of the casino or slightly in favor of the player.
A one-deck shoe is better for an advantage player because they can sit out when the odds are unfavorable and make very profitable bets when the odds are highly in their favor. But you can still make money by playing a lot of hands correctly in a eight-deck shoe.