Good question. A font is not an app; it is just a font installed on your device. After installation, it is part of the phone, and you can access it all apps. You need an app that supports selecting system fonts to use the font. For example, Apple Pages, Keynote, and Microsoft Word for iOS work well.
These apps further allow you to save PDFs of your writing. So you can write a letter and email your message set in the handwriting font to a friend or colleague.
