Job information in printed careers directories might be a year out of date by the time you read it.
Recruitment agencies charge employers a fee to find candidates. (Learn to spot agency ads - they usually refer to ‘my client’ without naming the employer.) Employers of STEM students rarely place opportunities with recruitment agencies.
Find a careers website that you can trust. Some websites - especially those less popular with employers - gather job advertising from other areas on the internet to appear well-populated with jobs. However, if employers don’t know that their vacancies are being advertised, they are not going to advise the website when the vacancy is filled or no longer available.