What does it mean when my iPhone says "Scanning Your Device"?

how do i prevent this from happening?

Ignore anything on the web that claims to detect an issue or help your iPhone......because it won't.

and how do i know if i got the software?

You tapped or clicked on a link.

and what did the scanning device actually do?

We don't know because we can't see your phone. But, you would be wise to run Safety Check on your phone.

How Safety Check on iPhone works to keep you safe - Apple Support

If you have recently downloaded an app (possibly a game), it is most likely the source of the bogus “scanning” message. Nothing is being scanned. I would delete the app, and if possible post a review of the bogus advertising tactics of this app.

All too common on sites that are less than legitimate. They are paid to pop up those fake ads and the purpose is to scare you into either calling a phone number where they will attempt to get your personal information or direct you to some software that you do not need. Don't visit those sites that are trying to take advantage of you.

It means you were viewing a website or using an application that attempted to serve you a badly programmed ad to sell you useless software.