My App uses several icons from the Asset Studio from the Android Studio.
When I'm selecting an icon it says: "These icons are availiable under the CC-BY license". The official CC Website tells me "You must give appropriate credit".
How and where (in my app) do I need to do so? Do I just need to add in a link somewhere in my app? Or what exactly are the conditions to use these icons in my app?
YES
According to the CC-BY license:
Attribution — You must give appropriate credit, provide a link to the license, and indicate if changes were made. You may do so in any reasonable manner, but not in any way that suggests the licensor endorses you or your use.
However, the Material Design Icon set readme states:
We have made these icons available for you to incorporate them into your products under the Apache License Version 2.0. Feel free to remix and re-share these icons and documentation in your products. We'd love attribution in your app's about screen, but it's not required. The only thing we ask is that you not re-sell these icons.