USB Drive – Does a USB boot CD protect rootkits?

I have an infected computer that I'm trying to diagnose. The attack is probably very sophisticated, and I do not know yet if it happened in the bios, the member, the active partition or any other space. The attack is underway, but I must know it while it is happening.

I plan to use a bootable USB operating system to diagnose the device.

I have not done it for a while, but what is the typical boot sequence if I try to load the operating system from a USB device?

The reason I ask this question is because, if my bios / mbr / other partition is infected, even if I load it from a USB stick, my computer and my information could still be compromised.

Thank you