for hsc purposes, in most cases where this is relevant, you would assume the statement is true. perhaps you dip into the stuff about 'domains' which was in some books (i remember it being somewhere in surfing physics).
if you want to know about the stuff at a subatomic level, i wouldn't go beyond the depth of wikipedia.
Yh in essence what @idkkdi said. They have magnetic domains which are initially unorganised. But then the effectively align with the magnetic field lines, when flux is passed through them. Hence the magnetic field is confined to a particular direction and area (more flux linkage) with the addition that the field itself is stronger...