Same way I know that Spotify Free exists. (ie, marketing outside of the App Store)

IIRC the iOS app promotes the desktop app, which can promote the premium plan.

I have to imagine a fair number of Google searches for "spotify remove ads", too, which can lead to an upgrade.

right, but their own app doesn't have the same restrictions. They can advertise the premium offering within the iOS app, they don;t need users to go further down a rabbit hole to find the premium offering.

I guess the crutch of Spotify's argument is that 30% is too high to charge while having their own competing app that isn't required to pay the same fees. Effectively using their unilateral control of the App Store and IAP to squeeze a third party competitor

Not every ios user has a desktop

Jesus. It's 2019. This isn't that hard. Is iOS the only way a user can discover facts about the universe?

Spotify can do this thing called "advertising". Look into it.

You’ve never even used Spotify have you ?

They advertise for Spotify Premium constantly on their free tier.

You can advertise Premium in-app. Just have to charge iOS users more than Android users.