If you are a software developer and love to make your apps and game then your bucket list may include making an app for App Store. The reason is there are chances of higher revenue generation. But if you are working on any kind of messaging app then you must be thinking about text message font as it must meet the default font of Apple. So, here is the answer to your question. iPhones and iPads use San Francisco as their default font.

This font is the exclusive property of Apple and is developed by themselves. The font was released in 2015 but introduced to iOS developers in November 2014. This was inspired by the favourite typeface of every developer sans-serif and falls under the category of neo-grotesque. 

Before launching their own first font all iOS devices used to rely on Helvetica. But later-on decide to launch their first-ever proprietary font inspired by two typeface Helvetica and DIN.
Apple has actually established serious guidelines to be followed by every app developer if they want to see their creation in the App store.

Here is the quick review of typeface size that must be followed by every developer to land in the App store.

In the same way, have set up guidelines for iPads. Apple wants everything elite in their product therefore they come up with this font and made it default. So, if you are planning in future to make an iPhone app than do keep in mind to use this font only and on the prescribed guidelines.

This is not the only if you want to add some texture you can check here the complete guidelines and tools designed to help app developers for making iOS apps within the set rules of Apple. They have their own font management system to make the process easy.

You can also use apple font tool suite to download fonts, user guidelines tutorials, an installer package.