The 5G Mobility Overview explains the 5G call flows for Intra-gNB and Inter-gNB handovers (PSCell Change) based on 3GPP 37.340 & 36.423.
The A3 measurement configuration is shared by the gNB to the eNB during NR leg addition and then the eNB sends it to the UE via RRC Reconfiguration. Once, the UE meets the A3 condition which means that another 5G cell within the same gNB is now a better candidate cell and has better coverage than the source cell then the UE sends A3 Measurement Report to the eNB. The eNB forwards that measurement report to the gNB over X2.
The gNB takes the handover/PSCell Change decision and sends a SgNB Modification/Change Required message to the eNB indicating a PSCell Change request to the target cell. The eNB reconfigures the UE to move it to the target NR cell and sends confirmation to the source gNB over X2 interface. The UE moves to target cell after completing RACH on the target cell.