PhD students are typically expected to TA one course per semester. The online web application HAT (https://www.lehrbetrieb.ethz.ch/hat) can be used to apply for a teaching assistant position each semester.
The assignment of a teaching assistant position can be organised beforehand internally (such as by speaking with the organiser of the course), but registration must still be done on the website.
Head TAs are expected to take care of the recruitment for the courses taught by our group; access to the system for this function can be provided by the administrative assistant.