How do I view the iOS app store on a mac? (OS X 10.11.6) The default way to install an iOS app was through iTunes. Having not used iTunes in several years, I'm trying to put an app on an iPad I have, and can't for the life of me find the iOS store in iTunes.

What am I missing?
You're not missing anything. Late in 2017 in iTunes 12.7, Apple removed iTunes ability to access the iOS App Store. Using iTunes 12.7, the only way to access the iOS App Store was via the iPhone itself. 

Many users disliked this change, as it's harder to navigate and review apps on iOS than it had been using macOS. A month or so later, Apple retreated and offered a version of iTunes that restores iOS App Store availability. This version of iTunes is 12.6.3, and you can read more about it on this Apple page. iTunes 12.6.3 can be installed even if one has already installed 12.7. The Apple page cited above contains a link to download iTunes 12.6.3.

I am very happy to have iOS App Store access back on macOS.