
Samsung Galaxy S6
The latest firmware for Samsung Galaxy S6 (SM-G920R7) is G920R7WWU3DRI2 with Android 7.0 (Nougat), released on September 9, 2018 for 1 region (CSC). Every official firmware build for SM-G920R7, newest first: one row per build with all regions (CSC) that received it.
Each firmware build has its own page at https://samfirm.shop/build/{PDA}, where {PDA} is the build number from the list above. Example: https://samfirm.shop/build/G920R7WWU3DRI2
SM-G920R7 started on Android 6 and has received 1 major Android upgrade; it now runs Android 7.0 (Nougat). The newest build, G920R7WWU3DRI2, was released on September 9, 2018. No new build has appeared for 98 months, so regular updates for this model have most likely ended.
Specifications of SM-G920F, a regional variant of the same device. Chipset and SIM lines are left out: they can differ by region.

SM-G920X

SM-G928A

SM-G925A

SM-G9298

SM-G935TZK
The newest firmware for SM-G920R7 runs Android 7.0 (Nougat): build G920R7WWU3DRI2, released on September 9, 2018.
The latest build for SM-G920R7, G920R7WWU3DRI2, was released on September 9, 2018 (8 years ago).
SM-G920R7 has had no new firmware for 98 months, so regular updates have most likely ended.
Yes, all firmware files for SM-G920R7 are 100% official, original, and obtained directly from Samsung servers.
To flash SM-G920R7, 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-G920R7.