explain differences between BIOS and CMOS

the CMOS chip is the 'configurable' part of the BIOS. When the
computer boots, the BIOS reads configuration information from the CMOS. The BIOS and CMOS are hardware and are not related to the operating system which is software.

The BIOS cannot be directly configured. It reads its configuration information from the CMOS.

The CMOS relies on a battery to 'remember' its configuration. If the battery fails, the CMOS will need to be reconfigured.

The POST settings are in the BIOS which does not rely on battery power