
Samsung Galaxy S6
Every official firmware build for SM-S906L, newest first: one row per build with all regions (CSC) that received it.
Specifications of SM-S906B, a regional variant of the same device. Chipset and SIM lines are left out: they can differ by region.

SM-S9060

SM-S9080

SM-S9010

SM-S902L

SM-S918Q
You can find the firmware on the model: SM-S906L page.
Yes, all firmware files for SM-S906L are 100% official, original, and obtained directly from Samsung servers.
To flash SM-S906L, you need to download the appropriate firmware, put your device into Download Mode, and use the Odin tool to load the BL, AP, CP, and CSC files.
Downgrading is only possible if the bootloader version (binary bit) of the old firmware matches the current one. Always check the binary version before flashing SM-S906L.