How and why is hydrogen bonding a type of intermolecular force and Van der Waals force?
A hydrogen bond is not exactly a type of van der Waals interaction; van der Wall forces include the Keesom force, the Debye force and the London dispersion force.
A hydrogen bond has a force of attraction much larger than that of the van der Walls force. The hydrogen bond is created by the interaction of hydrogen that is attached to an electronegative atom like flourine, oxygen, nitrogen with an electronegative atom that is part of another molecule.
A typical example of hydrogen bonding occurs in water, which is the reason behind water having a high boiling point and other characteristics distinct from similar compounds. The hydrogen bond has an acceptor and a donor. In water, the hydrogen that is attached to water is a hydrogen bond donor and the oxygen is a hydrogen bond acceptor.