The difference is small and pedantic:

“Once” means “A single time”

“Nonce” means “Used a single time”

Nonce carries extra information that something is being used.

Wikipedia claims this term dates to middle English:

Nonce is a word dating back to Middle English for something only used once or temporarily (often with the construction “for the nonce”). It descends from the construction “then anes” (“the one [purpose]”).

Nonce is used extensively in the context of cryptography to mean “used exactly one time”. Usually this refers to a random number that a server shares with a client to establish secure communications.

Can I leave all my drives connected, plug in a seperate SSD through USB, boot into Nobara live and install on that drive without it affecting my mint install?

Yes. Just double-check every part of the install process so you don’t write to the wrong device.

Also, if I do that will it put the EFI file on the seperate SSD?

Probably yes (depends on the options you pick during the install process). The external drive will get its own boot partition with appropriate EFI files. Then to boot from it, you would select the external drive in your UEFI.

I use rEFInd as my EFI bootloader: It lets me chain load other boot options (external drives) without touching my motherboard UEFI settings. I leave it installed to my main boot partition, but it scans for other bootable partitions at startup. Then it auto-populates a selector list of my main install, or whatever other external devices are plugged in. It can chain load GRUB, other EFI bootloaders, Windows, etc from these devices, so you don’t have to worry about compatibility with whatever bootloader the OS expects to use.

I agree with the person who suggested linking to Wikipedia articles.

For example, it’s not much help to learn that PCIe stands for Peripheral Component Interconnect Express, because nobody calls it that. As an outsider, now I’m wondering what a component interconnect is. It’s much more useful to link to a page that gives context about how PCIe is actually used.