What does $0 zero dollars mean in Apple app store terminology?

I have an app that I just noticed says I can buy it for 0 dollars.
 <https://i.postimg.cc/pdNq0Z0G/LANDRIVE-10.jpg> buy for $0.00

What does that even mean?

In normal operating systems, there's no concept of "purchasing" a
zero-dollar app, as that would just be called a free app (as in beer).

You don't purchase a free app.
You just install it.

But Apple has this concept of purchasing everything, I guess.
Even free apps.

But I've downloaded free apps before on the App Store.
And I don't remember it being called a zero-dollar app.

But this seemingly has a purchase price that I can "buy" it at.
But what does that blue 0-dollar button even mean in Apple parlance?