This is a Phase I/II study. The Phase I part will evaluate the safety and tolerability of sonrotoclax in combination with polatuzumab vedotin plus rituximab, cyclophosphamide, doxorubicin, and prednisone (Pola-R-CHP), using a standard 3+3 dose-escalation design, to determine the maximum tolerated dose (MTD) and recommended Phase II dose (RP2D). The Phase II part will assess the efficacy of the combination regimen in patients with previously untreated LBCL with high BCL2 expression or MYC/BCL2 rearrangements.