malware – How to securly create a Windows 10 bootable USB drive from a possibly infected system?

There is no foolproof way. Since you can’t trust the system, you can’t reliably verify the integrity of the image you will be using to create the bootable USB or, for that matter, of anything else. No matter what you do, it is possible for the malware to have interfered with it.

You could, however, do something the malware would likely not expect. Like make a bootable USB for a lightweight linux distro, boot live from it, and then make the Windows 10 bootable USB from thee live linux OS. It is quite unlikely for a windows malware to infect linux as well, but for that matter, it is unlikely for malware to infect bootable USBs in the first place.