sPHENIX measurements of collective behavior in small and large systems

sPHENIX, a next-generation detector at RHIC, completed commissioning in 2023-2024 and in 2024 recorded proton-proton (p+p) collisions at √sNN = 200 GeV and gold-gold (Au+Au) collisions at √sNN = 200 GeV. With large pseudorapidity coverage, full azimuth, and dedicated forward systems, the detector enables precise measurements of bulk particle production in addition to the high-pT jet and heavy-flavor program. These proceedings present the inaugural sPHENIX results for the transverse energy density dET/dη and charged-hadron multiplicity dNch/dη in Au+Au collisions, along with initial performance studies of neutral-pion elliptic flow and second-order event-plane measurements and long-range two-particle correlations in high-multiplicity p+p events. Together, these results validate the performance of the tracking, calorimetry, and global event-characterization systems and confirm the readiness of sPHENIX for a comprehensive QGP program at RHIC.