  A quantum system with a black hole accommodates two widely different, though
physically equivalent, descriptions. In one description, based on global
spacetime of general relativity, the existence of the interior region is
manifest, while understanding unitarity requires nonperturbative quantum
gravity effects such as replica wormholes. The other description adopts a
manifestly unitary, or holographic, description, in which the interior emerges
effectively as a collective phenomenon of fundamental degrees of freedom.
  In this paper we study the latter approach, which we refer to as the unitary
gauge construction. In this picture, the formation of a black hole is signaled
by the emergence of a surface (stretched horizon) possessing special dynamical
properties: quantum chaos, fast scrambling, and low energy universality. These
properties allow for constructing interior operators, as we do explicitly,
without relying on details of microscopic physics. A key role is played by
certain coarse modes in the zone region (hard modes), which determine the
degrees of freedom relevant for the emergence of the interior.
  We study how the interior operators can or cannot be extended in the space of
microstates and analyze irreducible errors associated with such extension. This
reveals an intrinsic ambiguity of semiclassical theory formulated with a finite
number of degrees of freedom. We provide an explicit prescription of
calculating interior correlators in the effective theory, which describes only
a finite region of spacetime. We study the issue of state dependence of
interior operators in detail and discuss a connection of the resulting picture
with the quantum error correction interpretation of holography.
