Hello. You are looking at the school demo site, school.demo.moodle.net
That is great but be aware that the mobile app is mainly for students and so is not really designed (yet) for teachers to add activities and edit courses. The school demo site is for the web version as well, and you will find you can experience all the teacher aspects better on the web version of Moodle. You can certainly do some teacher things on the mobile app - grade assignments, workshops for example.
If you want to try out new accounts and activities on the school demo site, you are most welcome to do so. You will need to use the manager account to make new users and courses and then feel free to try what you can - but better, on the web version if you are editing courses.