Cockroaches especially German cockies can enter the house in many ways, from tenants, cardboard boxes used for storage, fruit or veggie boxes, even transferred on clothing from unsuspecting friends.

They are hardy a species that breeds prolifically, getting into any crack, crevice, cupboard, microwave, white good, the list is endless.
Make sure all food boxes, second hand gear or any suspect item is thoroughly inspected before entering the house.

Keep food containers sealed and the general hygiene of the house kept in check.