where is retentive data is stored in PLC and how it is not lost during power failure???

There is a memory card in the PLC controller where all the data is stored. Earlier there used to be a battery in the controller which used to protect the data in case of power failure. But nowadays non volatile memory is used just like the USB in which the data does not get lost even in the case of a power failure

Memory card may be removed from controller. To prevent lost of data some PLC can use internal EEPROM to store some of data. Other can use capacitor to save RAM data for long time(for example 5 days).

In a SIEMENS S7-300 system can safeguard data securely.
By instructions SFC51 system, you can control the battery status and schedule back data from a DB (RAM) to a DB (unlinked) in the MMC.

properties:
UNLINKED:
Data blocks with UNLINKED property only stored in the load memory, but do not occupy space in working memory and not linked into the program. You can not access them with MC7 commands. The contents of such DBs can be transferred to working memory only with SFC 20 or SFC 83 BLKMOV READ_DBL depends on the CPU.

READ ONLY:
Write protection for data blocks, data can only be read but not modified.

You can control and manage information from DB's adjusting its properties in the statement. In my case every Sunday at 2:00 saved data for a backup date in the MMC (EEPROM).

Flash memory or battery backed RAM. From my experience, the largest issue with losing retentative memory values is not power loss, but program loading with overwriting of state RAM with earlier values. It is important that proper procedures be followed i.e. saving state RAM prior to program loading.