I loved that one

That is where you have the the undo segments. So every time you start a transaction a undo segment is assigned to the Txn and it has the snapshot that guarantees read consistency. This is called MVCC in databases. This is how read statements get the correct fat when your transactions have an intent to write. Also as the transaction progresses and you make data changes (dmls) a time snapshot (SCN or CSN) is also maintained in undo and also at times this is written to redo ahead of time about every 3 seconds or other scenarios. Basically to put all these together redo or wal and undo are used for crash recovery. When instance starts it reads the redo and undo to see which statements were commuted but not written to disk and write them to disk. Which are the statements that are not committed that are in redo and sometimes even on the disk. You undo those changes and update the checkpoint. You can go deep. Oracle does this best. That is the reason oracle is what oracle is! When isolation, consistency and durability is the concern there is a reason banking systems and critical systems use oracle or mssql databases and pay for them rather than open source ones.