It is probably worth remembering that Spanish has a specific rule stating that "de" must always appear between "un millón" [or "millones"] and the noun which it is qualifying. Is this mentioned in one of your lessons? ... Does the same rule also apply to "billón"?
Freeform Writing Exercise B2
Yes, we have an A2 lesson on "large numbers" mentioning this.
I hope it's useful.
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