
Samsung Galaxy S20 Ultra 5G
The latest firmware for Samsung Galaxy S20 Ultra 5G (SM-G988Q) is G988QOPU1ATG9 with Android 10, released on July 28, 2020 for 1 region (CSC). Every official firmware build for SM-G988Q, 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/G988QOPU1ATG9
Specifications of SM-G988B, a regional variant of the same device. Chipset and SIM lines are left out: they can differ by region.

SM-G988W

SM-G981V

SM-G986W

SM-G985F

SM-G980F
You can find the firmware on the model: SM-G988Q page.
Yes, all firmware files for SM-G988Q are 100% official, original, and obtained directly from Samsung servers.
To flash SM-G988Q, 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-G988Q.