Apple is renowned for its walled garden which has its upside in terms of security but prevents users from accessing apps that are not on the App Store. Thankfully, Apple opened a small gate in the fence a couple of years ago which allowed open source applications to be signed and installed by the user for personal use. This opened the door for the sideloading of apps like Kodi and emulators like nds4ios which would otherwise never make it to the App Store.
The long way to do this would be to download the source code for the app, compile it in XCode and then transfer the app to the phone. The catch here is that you need a Mac for it or in the least a Hackintosh, besides some patience. However, this is also the only way to run all available open source applications, a handy list of which is available at this link. The official guide to compiling Kodi can be accessed here.
A much easier option is to use Cydia Impactor. It is much less time consuming and easier to execute, as illustrated in this guide. Again, the rider here is that you need the already compiled IPA files to sign them with your own certificates. Many of the prominent ones including Kodi and most emulators are available here.
With Apple, there is always “one more thing” and it is true here as well. The free developer certificates last only a week, so it means that the process of sideloading the app has to be repeated every week. It is definitely an inconvenience but by design as it reduces the scope for piracy and bypassing of the App Store. The pain can be alleviated if you go for a paid developer account ($99/year) which allows the app to be installed till the certificate expires at the end of a year. Lastly, if you fancy coughing up $10/year, then you can go along with the BuildStore which allows you to directly install select apps from the browser with the certificates lasting for nearly a year.
Of course, apart from accessing Kodi and emulators, this option also allows one to create and test one’s own apps without coughing up anything which is incentive enough for me to have a go at creating an iOS app as well for the website.