“Player X over 245.5 passing yards” is a prop. Because each book prices props with its own model and its own liability, the same prop is frequently posted at different numbers at different books - which makes props the most fertile ground for line shopping, middles, and devig-based +EV scans, and also why prop limits are lower than game-line limits.
ParlayAPI serves 4,000+ distinct prop market keys in a typical 24 hours across its covered sports, and archives 30M+ player-prop closing lines (since June 2022) for backtesting prop models against the close. Browse the live keys at /v1/meta/markets.