Your description of the steps you’ve taken is inconsistent with the result you are seeing which sounds like an initramfs prompt. Windows doesn’t use busybox or initramfs, so if you are seeing that, then you did not wipe all of the drives as you claim.
If you want to use Ubuntu to wipe your hard drives, you can boot the Ubuntu installation media from a USB and choose “Try Ubuntu”. Then you can open the “Disks” application to format each hard drive. This will effectively erase everything on those drives.
Simply choose the disk on the left and choose “Format” from the drop down. Since you are planning to install Windows, choose “GPT” as the partition scheme when it asks.
If you do this for all hard drives, it will not be possible for any remnants from Ubuntu to remain. You will have a blank slate.
As far as installing Windows, that is off-topic and if you continue to have Windows installation issues, you can ask about that on SuperUser.
Your issues accessing BIOS are unrelated to any operating system so it has nothing to do with Ubuntu. BIOS is your motherboard’s firmware. Generally you need to press a key immediately after power-on to access BIOS settings. Usually vendors will notify you which key to press on the splash. In case that is disabled or you are unsure, read the documentation for your computer’s motherboard. All motherboards are different.
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