How do you mark the beginning of backup and what happens after that? | Oracle DBA

You can use the following statement to mark the beginning of backup:
SQL>alter database begin backup; The preceding statement marks the beginning of a backup by freezing the header of the datafiles. After that, the files cannot be changed. The changes in the data will be recorded in the files after the backup is complete.
You can indicate the completion or end of the backup using the following statement:
SQL>alter database end backup;