Hello, as a researcher I made research trips that my university reimbursed 100% a few days later. Should these reimbursements appear in my W-2? Or anywhere else?

You expense reimbursements are probably not reported on your W-2, as they are not considered income.

You do not need to include these reimbursements anywhere as income, as the money you received was to reimburse you for expenses you incurred and paid on behalf of the university.