  The aim of this dissertation is to clarify the structure of entanglement, a
type of quantum correlations, in various quantum systems with a large number of
degrees of freedom for holography between generic quantum systems and
spacetimes toward a quantum description of our universe. Previous examinations
of entanglement and holography have focused on specific classes of quantum
systems due to the lack of computational techniques in field theory and the
inherent limitation of holography. This dissertation informs various methods
and formalisms to overcome these difficulties by extending the target quantum
systems with mass and interactions, boundaries, and geometric variational
ansatze. These approaches provide insights into the generalization of
holography from the bottom up. This dissertation initiates a comprehensive
study beyond conventional holography by establishing new techniques in quantum
field theory, holography, and tensor networks. Focusing on entanglement
entropy, we found it is generally expressed in terms of renormalized two-point
correlators of both fundamental and composite operators. Beyond entanglement
entropy, we found the operational meaning of the entanglement structure in
generic tensor networks. Furthermore, we established a correct prescription for
the AdS/BCFT correspondence with a local operator quench.
