I am aiming to do a PhD in Machine Learning and in Germany since I have a Masters in ML already I wanted to know what are the best options to aim for ?
University websites might be a good way to search a suitable laboratory for you since you have a topic in mind. You may go through the current researches and research interest of the laboratories and professors. Sometimes, the university may even publish a list of available projects and/or supervisors who are looking for Ph.D. students. Try contacting the supervisors through email to discuss your ideas and the opportunities.
Another way is to search for Ph.D. opportunity through your undergraduate and master supervisor. They might have a lot of connections with other universities as well as recent researches. They may guide you based on your knowledge and interest.