
Samsung Galaxy S5
The latest firmware for Samsung Galaxy S5 (SM-G900F) is G900FXXS1PPCG with Android 5.0 (Marshmallow), released on March 14, 2016 for 1 region (CSC). Every official firmware build for SM-G900F, newest first: one row per build with all regions (CSC) that received it.
SM-G900F started on Android 4 and has received 1 major Android upgrade; it now runs Android 5.0 (Marshmallow). The newest build, G900FXXS1PPCG, was released on March 14, 2016. No new build has appeared for 129 months, so regular updates for this model have most likely ended.
Box codes of SM-G900F for different colors and markets. Firmware is chosen by the model code SM-G900F and your region (CSC), not by the SKU.

SM-G935TZK

SM-G930AZK

SM-G935AZS

SM-G935TZS

SM-G935AZK

SM-G935TZD

SM-G975WC

SM-G975FC
The newest firmware for SM-G900F runs Android 5.0 (Marshmallow): build G900FXXS1PPCG, released on March 14, 2016.
The latest build for SM-G900F, G900FXXS1PPCG, was released on March 14, 2016 (10 years ago).
SM-G900F has had no new firmware for 129 months, so regular updates have most likely ended.
Yes, all firmware files for SM-G900F are 100% official, original, and obtained directly from Samsung servers.
To flash SM-G900F, 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-G900F.