Why the redo.logfile in my standby db flash_recovery\dbsid\backupset\ ?

1)I use rman script"duplicate target database for standby nofilenamecheck dorecover"
why the redo logfile automatcly generate in my standby db flash_recovery_area\sid\backupset?while it is in oradata\sid fold on primary role databse?
2)does dataguard apply archivedlog transmitted from primary db directly?or by standby redo log or standby log?
3)How and why standby redo logfile used?

the sequence# 1 has the fisrt_change# greater than
the next_chage#, it's any reason for that?.
SEQUENCE# FIRST_CHANGE# NEXT_CHANGE#
1 534907 551724
I was confused?????
SQL> select 534907-551724  from dual;
534907-551724
       -16817

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