Considering that all modern motherboards have code signature procedures in place like it is mentioned here: Is it possible for malware to be in the BIOS or in hardware? I was wondering is it safe to just download BIOS firmware from anywhere on the net or using HTTP? Is it possible to install an infected BIOS firmware on a motherboard produced 2013-2020 and the motherboard code signature not detect it?
Let's admit that everything is possible. It's called zero-day attack. The serious matter to discuss about is if this possibility is high or low. It's very very low. Every motherboard company has a regular penetration testing procedures for its products. Although, for older -not currently supported- motherboards for sure there will be exploits (bios rootkit attack most common) and you should think that these exploits will be probably detected by the antivirus, so the risk is if your computer is really old, with old and not supported operating system. In general, it's really unlikely that you find a malicious bios firmware for motherboards especially for 2013-2020 ones, and more unlikely this malicious firmware will be undetectable. A real danger tho will be if someone purchase an exploit from the dark web and target the victim.
Check that out about bios rootkit attacks: https://searchcloudsecurity.techtarget.com/definition/BIOS-rootkit-attack
Methods of preventing BIOS rootkit attacks include:
Implementing digital signature technology to prevent unauthorized access. Making the BIOS non-writeable. Burning a hardware cryptographic key into the BIOS at manufacture that can be used to verify that the code has not been altered. If an unauthorized BIOS-level rootkit is detected, the only way to get rid of it is to physically remove and replace the memory where the BIOS resides.