  Loops are one of the fundamental structures that trace the geometry of the
magnetic field in the solar atmosphere. Their evolution and dynamics provide a
crucial proxy for studying how the magnetized structures are formed and heated
in the solar atmosphere. Here, we report on spectroscopic observations of a set
of transition region loops taken by the Interface Region Imaging Spectrograph
(IRIS) at Si IV 1394 \AA\ with a sit-and-stare mode. The loops are
corresponding to the flux emergence at its very late phase when the emerged
magentic features in the photosphere have fully developed. We find the
transition region loops are still expanding and moving upward with a velocity
of a few kilometers per second ($\lesssim$10 km/s) at this stage. The expansion
of the loops leads to interactions between themselves and the ambient field,
which can drive magnetic reconnection evidenced by multiple intense
brightenings, including transition region explosive events and IRIS bombs in
the footpoint region associated with the moving polarity. A set of
quasi-periodic brightenings with a period of about 130 s is found at the loop
apex, from which the Si IV 1394 \AA\ profiles are significantly non-Gaussian
with enhancements at both blue and red wings at Doppler velocities of about 50
km/s. We suggest that the transition region loops in the very late phase of
flux emergence can be powered by heating events generated by the interactions
between the expanding loops and the ambient fields and also by (quasi-)periodic
processes, such as oscillation-modulated braiding reconnection.
