A transformer is not born with "primary" or "secondary" sides. A transformer is not born as a step-up or step-down transformer. It works both ways.
It has high voltage "HV" and a low voltage "LV" sides.
The most typical is to connect the power supplying side from a higher voltage, so the HV side becomes the primary side.
But it is entirely possible to connect it the other way around so that it increases the voltage if that is necessary for the application. In that case the LV side becomes the primary.
What is important is that the HV and LV sides can be identified by the terminal labels. On the name plate it says that the HV side is H1-H2-H3 and the LV side is X1-X2-X3.