There are many options in terms of kits, templates and mockups to use when looking for a correct design of a mobile app, as well as a web page.

You have a lot of variety of icons, illustrations, and more elements in the Sketch page, where you can also see variety in pre-designed models of interfaces according to the operating system of the mobile, be it ios or android.

These templates and mockups you can find are quite decent and comfortable for users, which is important when offering a good service to an audience, a good interface can make the difference whatever the app is, an online store, a mailbox, a social network, etc!