What is this file?

There are no viruses that affect macOS so just forget that idea.

I share this computer ...

Sharing a Mac is ok. Sharing your login credentials is not ok. If you allow others to use your Mac, set them up with their own User Account login credentials: Add a user or group on Mac - Apple Support. Never divulge your Admin password to anyone else under any circumstances.

I don't quite understand your question. The percentage of whatever was downloaded is not relevant. The point is that nothing in iCloud Drive (or a Mac for that matter) is the product of spontaneous generation.

Don't share your Apple ID credentials either: Security and your Apple ID - Apple Support

Tiger_18 wrote:

So it was 100% downloaded and out there on purpose?

Stop sharing your Mac login. That’s always trouble, and trouble well past this (benign) case.

Whether this was done on purpose is another discussion. It’s easy to create these files, whether deliberately, or by fumble-fingering an operation.

It's understandable to ask. It is completely possible to be doing something on the Internet and something gets downloaded as you're clicking around various web pages looking for something else. Most of the time, whatever gets downloaded is a web page that uses all manner of scare tactics whose ultimate goal is to get you to spend money.

The content of whatever got downloaded — p**n or whatever — is completely irrelevant. The underlying threat is to motivate you to take some action that is not justified and not grounded in factual knowledge.

The only factual knowledge truly useful in that regard is that the Internet resembles the mythical wild west where there are no rules, almost literally no laws, and in which the lowest and basest form of humanity thrives.