Assume a USB drive has an MBR or GPT that has malware in it. I already understand that booting from such a USB device would execute the malware as part of the bootloader. If I understand correctly, if a clean computer was booted into a traditional OS (macOS, Windows 10, Ubuntu, etcetera) and then the infected device was inserted then the OS would not execute code but would still see the MBR or GPT.
Is it correct that
A: an infected MBR or GPT introduced to a clean, already-running OS would have no malicious effect at that time...
And B: would have to remain inserted during a reboot (and be the booted device called by the BIOS) in order to have a malicious effect?
Please note that this question is limited in scope and does not pertain to the firmware layer of the USB drive, nor to the data on its data partition. IMO those questions should be handled discreetly, not conflated.