How does an update statement work at architecture point of view? And where do the rollback segments

Hello team,

Please help me in understanding of the internal mechanism "How does an update statement work?, And I heard that the copy of "Before Image (BI) " is kept in the SGA, and oracle allocates an empty buffer.

where do the rollback segments present? In the SGA or in the physical undo dbf files? Also explain how do we get ora-01555 snapshot too old error?