When you give alter tablespace begin backup command, checkpoint occurs for all datafiles in the HOT BACKUP. the datafile header checkpoints are advanced to the SCN captured when the begin backup command is issued. And this SCN goes in the backup file as well.

Alter tablespace end backup command creates a redo record containing the begin backup checkpoint SCN. Also the datafile checkpoint is advanced to the database checkpoint.

So from this redo record and the initial SCN in the backup datafile, oracle knows which all redo needs to be applied to the restored backup file to make it consistent.

(correct me if im wrong)