Samsung Galaxy S20 5G
Every official firmware build for SCG01, 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/SCG01KDS1DXB1

SCG04
You can find the firmware on the model: SCG01 page.
Yes, all firmware files for SCG01 are 100% official, original, and obtained directly from Samsung servers.
To flash SCG01, 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 SCG01.