Yes, M1 chip.

I asked in the Apple Community and someone told me that by ejecting the software and trashing the installation icon that I'd be clear. Oddly, that's how I first knew there was Discord there--I found the installation icon in the trash yesterday, never having seen it before. So I should be good, but am still mystified about how some software could be there without consciously installing it. Time to call the neurologist?

On an M1 or newer chip, many iphone and ipad apps run fine.
Some are gorgeous on a large monitor, and some are dreadful, and some just don't work.

Choice is good.

I belong to a few groups on it that discuss electric vehicles
I would be concerned about using it or acess to it on your computer

Not on my phone.
I am unaware of ever having used it, and no one else uses this laptop.

Bizarre!

There is an official discord application for macOS over at https://discord.com/download

If I were you I'd download that and the compare it to the application you've got. You've got
quite a mystery as to why it showed up.

The 'external drive' thing you noticed is pretty normal. Software is often distributed in
virtual disk "images" that look like an external disk. You can run things on them .. normally
an installer .. and then eject them and throw away the disk image. The disk image file
is usually something that ends in ".dmg" for disk-image.

In any event, I'd suggest deleting it. It's quite possibly a trojan horse malware app pretending
to be a discord app to get people to try and run it.

Note there is an application store for Mac apps. Those are pretty well vetted, and random
applications (like the discord app) are not nearly as safe to run.

Yes, all this makes sense. I use Malware Bytes and it detected nothing. I am so careful about not opening links that are in any way suspicious etc. but this must have gotten past me. I kind of wish I'd taken a screenshot, but it creeped me out so much.