Major work can suspend parts of the cover
A standard homeowners policy assumes a finished, occupied dwelling. Significant structural work, an open roof or a house emptied for the duration changes both the risk and the occupancy, and policies contain conditions about each. Builders risk cover exists for exactly this period and it is a different product rather than an endorsement people happen to have.
Materials on site are not building yet
Lumber, fixtures and appliances delivered but not installed sit in an awkward category. They are not part of the structure and they are not ordinary contents. Theft from a work site is common and whether anything responds depends on how the cover was arranged.
The contractor insures the contractor
A contractor’s liability policy covers their negligence. It does not cover your building, and it does not make their certificate a substitute for your own cover. Asking for the certificate is still worth doing, because a contractor without one is an exposure you absorb.