Used Macbook Pro 2018 - what is the safest approach to ensure no virus?

I bought a MBP 2018 off ebay.

When I boot up the machine it showed the install prompt for Big Sur. I imagine the person who owned it before reset it such that it was ready for re-installation.

My first question is, how sure can I be at this point that there is no malware on the machine?

I went through the installer and then I downloaded Monterey via the app store. I put Monterey on a usb drive and made that drive bootable.

If I can't be sure of the first question, if I install Monterey via the usb drive that I created using the os I am unsure about, can I be sure the bootable usb drive that has Monterey is ok?

Thank you!

Boot from your USB, then use Disk Utility to Erase the existing Volume, after that you can re-install mac OS. Anything the previous owner would have installed would be wiped this way.

If you want to go the extra mile use another machine to create the bootable installer or use Internet Recovery (boot with Option-Command-R).