How do I download apps in ipad mini ios9.3.6
It is possible to download older versions of an app from the iOS App Store, but you must already have "bought" (or used "Get" for a free app) the app using the Apple ID used by your older iPad.

Once the app is registered as Purchased (Apple calls it this even if it's a free app) against your Apple ID, you can go into the Purchased tab on the App Store on your older iPad and download from there.

You will get a pop-up "Download an older version of this app?" ("The current version requires iOS X.X or later, but you can download the latest compatible version") where you choose Download.

So to be able to do this, if you haven't already got the app, you do need a second iOS device using the same Apple ID for the App Store.