I wanna customise my Pages app to add some more useful features to me than the one existing in the original Touch Bar. However I don't wanna start from scratch and use some of the feature already existing in the original Touch Bar in pages (for example putting in bold, underling, etc.).
Do you know where can I find the setup of the original Touch Bar in order to modify it ?
It is not possible to modify the system Touch Bar, so adding buttons is not possible. You can always only replace the Touch Bar with a BTT specific one.
There are some visibility options you an use to modify the behavior per-app: