Understanding the seismic precursors is essential for deciphering earthquake rupture physics and can aid earthquake probabilistic forecasting. With regional dense seismic arrays, we identify seismic precursors of 527 0.9 ≤ M ≤ 5.4 events of the 2019 Ridgecrest earthquake sequence, including 48 earthquakes with series of precursors. These precursors are likely immediate-foreshocks that are adjacent to the earthquakes. Their corresponding precursory signals share high resemblances with the earthquake P-waves and occur within 100 s of the P-waves. However, attributes of the immediate-foreshocks, including the amplitudes and preceding times, do not clearly scale with the eventual earthquake magnitudes. Our observations suggest that earthquake rupture may initiate in a universal fashion but evolves stochastically. This indicates that earthquake rupture development is likely controlled by fine-scale fault heterogeneities in the Ridgecrest fault system, and the final magnitude is the only difference between small and large earthquakes.