The PHB says the basic use of the Wish spell is to duplicate the effect of any other spell of 8th level or lower without having to meet any requirements of costly components.
If, as a wizard, you wanted to use Wish to cast (for example) Mordenkainen's Magnificent Mansion, does that mean that as well as using a 9th-level spell slot to cast Wish, you would also have to use a 7th-level spell slot (since Mordenkainen's Magnificent Mansion is a 7th-level spell)?
No. Wish uses a 9th level slot for any casting, and one of its canonical uses is to duplicate any spell (of 8th level or lower). Consider the case of a 19th level Wizard who, for one reason or another, has expended all of their 7th and 8th level slots, and further, doesn't actually have Mordenkainen's magnificent mansion prepared -- but wants to cast it in order to take a safe long rest, recover their slots, and incidentally make a couple small changes to their prepared list.
They have wish at their disposal, and a 9th level slot -- if casting wish required also having a slot of the level of the spell it's duplicating, we'd have a case of the most powerful and versatile spell there is, being unable to do one of the things it's designed to do.
You would only be using the 9th lv spell slot. The wish spell does not ask you to expend any other slots.
"The basic use of this spell is to duplicate any other spell of 8th level or lower. ... The spell simply takes effect."
