Sportsbooks make money mainly from the vig — the margin built into every price — plus by balancing action and limiting the bettors who beat them.
On a balanced two-way market priced 1.91 / 1.91, the book takes bets summing to ~105% of a fair market. Whatever happens, it keeps roughly the overround. Multiply that small hold across millions of bets and it's a large, steady profit.
Books try to attract roughly equal money on both sides so the vig is locked in regardless of result. They also shade lines toward popular teams and favorites, because recreational bettors overbet them — extra margin on top of the vig.
Books profit from recreational volume, not from being unbeatable. Sharp bettors exploit slow soft-book prices, line shop for the best number, and bet +EV against the fair line. Books respond by limiting winners — which is why coverage and speed matter.
Largely yes — balanced action locks in the vig with no result risk. But they'll also take a position when they think a line is off.
A small minority do, through line shopping, +EV betting and discipline. Most bettors lose to the vig over time. Winning gets your account limited.