minni
A -- I'd ask on Hardware. My understanding of external USB drives stops at "plug it in and see if it works". There may be an issue with the partition table on it, or it may actually be failed (always a risk with e-bay hardware)
B -- It doesn't make sense to buy a huge USB drive if you're only going to have a single operating system on it. That's unfortunately a limitation of using something like Rufus or Etcher to install it. If you wanted to try Ventoy (https://www.ventoy.net/en/doc_windows_cli.html) you could use a single bigger drive, but as I said above I've never used it on Windows and I think it's a bit finicky - not a route I'd send somebody down without support if I can help it.
C -- my mistake. You should have backup media for the system you intend to install Linux on. If that's the Dell, then make that backup instead of the Lenovo. I would only install on one of your systems at first - make sure it works for you, and it's less work to undo if it doesn't.
D -- yes. you only need a single copy of the Windows 11 boot media. It can be reused any number of times.
E -- external drives do have partitioning, just that it's usually hidden from the user. That's actually how Ventoy works: it turns a USB drive into an OS install with hidden partitions (total about 1GB of space used), and gives you the rest of the space back as a regular USB drive. the Ventoy software scans the "regular drive" part of the USB drive for ISO images and other bootable images, and generates a menu to let you choose which one to boot every time you use it. With a tool like that, it would make sense to have a big USB drive, as you can copy dozens of ISO images on it to try everything under the sun if that's your fancy.
F -- yes, I mean the laptop. I use the more generic "machine" because not everybody uses a laptop. 🙂
G -- using a tool like Etcher or Rufus will "burn" the ISO to the USB drive. But they do that by copying the ISO to the drive, overwriting the existing partition table. Essentially, they resize the media from however many GBs you actually have to the size of the ISO. It's possible to make a 2nd partition with the remaining space on the drive, but as far as I know neither of those tools will actually do that. This is why it doesn't make sense to use a large thumb drive to make boot media for an ISO installer.