To get to your development_behaviour_packs folder paste the below into Windows Explorer:
%userprofile%\AppData\Local\Packages\Microsoft.MinecraftUWP_8wekyb3d8bbwe\LocalState\games\com.mojang\development_behavior_packs\
Download Vanilla Behaviour Packs here.
The Finished Example Behaviour Pack Is Here. To edit this you can rename the extension from a .mcpack to a .zip which you can then edit. Make sure to zip it back up and rename again afterwards though.
You can also watch my YouTube tutorial below: