Which is better for a university student who wishes to complete postgraduate studies: working in a governmental or private institution, or working in trade with the possibility of combining it with studies?

All halal gains are good, and some are better than others due to specific considerations. Trade may be better than government or private employment, and a job may be better for some people, such as those who make their job a means of serving people. It is better for you to work in something that you can reconcile with your studies, especially if your specialization is beneficial to the Ummah.