joy Well, will try to answer as short as possible
Boot 1, Boot2, User Area, RPMB = configuration of the Hardware Partitions that by default is set during manufacture of the eMMC. Boot 1 functions as a place for bootloaders with a minimum of boot code, Boot 2 is a backup of Boot 1, mostly empty, only use in OTA when device need to update their bootloaders.
User Area, is a place for general Software Partitions needs by Android, Vendor and User.
Ext_CSD, as a file is general boot configuration and boot bus conditions of the emmc itself.
You can learn more by reading this article.