Insert timer into video panel

Hello,

I want to create an app like the "Timer"app provided in the app store but with custom features. Of course the main feature should be to display a timer in the video panel.
I’ve read through the docs and the forum and just found the answer “Use the (meeting) SDK.”.

Are there more detailed instructions on how to display something in a video tile in the app or even to add an additional video tile containing custom content?