I think it's great that you're thinking about this now! Many people don't think about this until it's too late and they're already trying to find a job.
The short answer is "all of the above." The broader you cast your net, the more opportunities you're likely to catch!
Here's some more specific info from my own experience:
- Talking to professors: When I was studying Materials Engineering in college, I had a few internships during the summers. In every case, a professor helped me get the job in some way. In some cases, they knew of a contact in industry who was looking for a good student intern, so all I needed to do was ask!
- Internships: Following up on the point above - internships are a great way to get your foot in the door with a certain job/company. It's a great low risk, short-term way for a company to see how you'll do - if you do well, there is a good chance that they'll ask you to come back for a more permanent position once you're ready!
- Applying to job postings: The first job I got out of school was actually through applying to a job posting online. It's not usually the most effective job search technique, since there can be hundreds of people applying for the same job. But it does work in some cases!
- Using your network: This is probably the most important. You pointed out that you couldn't find many job postings online - this may be because many positions are being filled by companies reaching out directly to people they think may be right for the job. (I got my second job this way.) Employers may prefer this approach to opening up the job posting to the broader public and having to sort through hundreds of applications. The most important thing is just to make a good impression on everyone you meet, and it's always possible that they'll end up helping you out sometime in the future!
So in summary, don't be discouraged by not seeing many postings online. There's plenty of other ways to get a job! Just focus on meeting lots of people and making good connections while you're in college!
PhD, Materials Science and Engineering