Your student loan servicer is assigned at the same time your loan is disbursed to you. You should see various communications coming from the new servicer directly to your school email or the email you have affiliated with studentaid.gov.
You can always register and login at studentaid.gov to verify your servicer from the federal servicer listing.
You may also have private student loans, which are disbursed from any private lender or bank not affiliated with the federal government. These lenders will also reach out to you at the time of loan disbursement.