In 5G, SSB is the broadcast channel used by the UE to estimate RSRP and thus, it is used for all access related and mobility related decisions.
Some implementations have single SSB while others have multiple SSBs and in case of multiple SSBs, the concept of Beam Sweeping comes into play.
This session explains that concept and how it works!
A simple explanation of 5G SSB Beam Sweeping - Ref: 3GPP 38.802
