How do I scan my iPhone XS Max with free malware?

If you are referring to Malwarebytes, the iOS version does not scan for malware.

From the description on the vendor's own site:

Malwarebytes – iOS

"The malware scanner isn’t available on iOS. It is not possible for an iOS app to scan for malware. Fortunately, the risk of getting malware on an iOS device is low and Apple has a stringent review process regarding app admission into the App Store."

From the App Store description:

"Virus Cleaner on iOS: Due to iOS limitations, direct virus cleaner scans for malware files aren't possible …"

Although the iOS app has a 7-day free trial period, after that you must pay subscription fees to unlock most of the features.

There is no way to "scan" your iPhone for malware, as your iPhone isn't even susceptible to them. If you see apps that claim to do so, they are lying and do not. They just want to take your money or personal information, while pretending to scan in the background.

Even if you can find a "free" app to scan your phone, it is still not worth the money.

This is a scam. The app will find something "suspicious"....even though it cannot really scan your phone. Then, you will be offered software......which will not be free.....to fix a problem that does not exist on your phone.

The scammer wins. You lose.

You don’t.

lelefjc (Author)

Well there had been a macOS version that seemed to check for malware.

The 'Malwarebytes for Mac' used to be OK, & still scans for bits of stuff.

And their iOS mobile version that says 'Free - Offers in-App Purchases"

I've not tried this; and so have no idea of its worth..

Malwarebytes - Mobile Security on the App Store

//apps.apple.com/us/app/malwarebytes-mobile-security/id1327105431

The App in-store preview has user comments, nothing outstanding nor recent.

So that may be OK, IF you didn't have to Buy anything; to find it wasn't worth it.