How Production DB and Standby DB Update as Same Data?

Archive means if you made any changes in your database like some insert update delete query. this changes will save in redo log buffers ... once one 3rd of this buffer fills or every 3 seconds log writer will write these details in redolog files in a database 2 redolog files must if one redo log file fills log switch will happen. It will write in another log file if db is running in archive log mode this filled redolog files wil be archived by archiver processer if system is running in no archive log mode. It won't archive and it will simply overwrite.

please correct me, if i am wrong.

Your Database must be in archivelog mode if you are using standby.

As you telling, you have backup before 6 hours,up to that point it is ok. But at the time 3:10 PM, your database must be in sync mode.