Sharepoint - Migrating SharePoint 2013 apps to 2016

Take this as some form of documentation. Turns out it was because I was using host named site collections, and upon mounting, the web application default url that I mounted it to had the same URL, so it thought it was a path based, which made the web application work fine, but the apps weren't registering.

If you are migrating a host named site collection, make sure that you mount it from a base web application, with a root site collection. I can't take credit for this though. My colleague pointed me in the right direction.

• Create App Management and Subscription Settings Service Applications with the same restored service dbs from 2013.
• Set-SPAuthenticationRealm -Realm "<guid>" where the guid is the same spauthenticationrealm as the 2013 farm
• Important....You MUST use the same web application URL. The webapplication url I use is irrelevant because the root just hosts all the HNSC. I tried changing it after the fact and it seemed all okay.
• Create new app site catalog and install/upload all the apps
• Mounted the Team Site content db (did this first since this is the root HNSC)
• used $spsite.Rename() to change the HNSC URL to the test url
• ((Get-SPSite <siteurl>).ContentDatabase).RefreshSitesInConfigurationDatabase()
• iisreset on web front ends
• Configure App URL if you haven't already (should be a forward lookup zone that points to your FE or a VIP to both/all front ends.

My application came up and the apps were all there with all of the apps and app parts showing just fine. If you use the ...(ellipis) on the App in the Site Contents, you should see the "About" section.