It would be best if you start looking for employment while you are still at university, during the last semester, to get an idea of where to begin, what to look for and expect, and what your options are.
You will be able to find employment opportunities on online job advertising websites, university notice boards, or even on company websites and social media. If you already have created preferences, meaning there is a company you would like to work with, you can email the company and ask whether there are any open job positions available or whether they are planning to hire in the future. The good thing about an extended residence permit after studies is that you can work any type of job while seeking contractual employment related to your field of study.
Advice: Keep in mind that graduates who have German language skills will likely be given priority over those who do not. So during your time at university, even if you are studying in English, pay attention to your German language skills. Knowing German will come in handy not only when it comes to finding employment, but also obtaining a settlement permit more quickly, as well as socializing, networking, and finding your way around Germany without any barriers.