  In recent years, a new generation of optical intensity interferometers has
emerged, leveraging the existing infrastructure of Imaging Atmospheric
Cherenkov Telescopes (IACTs). The MAGIC telescopes host the MAGIC-SII system
(Stellar Intensity Interferometer), implemented to investigate the feasibility
and potential of this technique on IACTs. After the first successful
measurements in 2019, the system was upgraded and now features a real-time,
dead-time-free, 4-channel, GPU-based correlator. These hardware modifications
allow seamless transitions between MAGIC's standard very-high-energy gamma-ray
observations and optical interferometry measurements within seconds. We
establish the feasibility and potential of employing IACTs as competitive
optical Intensity Interferometers with minimal hardware adjustments. The
measurement of a total of 22 stellar diameters are reported, 9 corresponding to
reference stars with previous comparable measurements, and 13 with no prior
measurements. A prospective implementation involving telescopes from the
forthcoming Cherenkov Telescope Array Observatory's northern hemisphere array,
such as the first prototype of its Large-Sized Telescopes, LST-1, is
technically viable. This integration would significantly enhance the
sensitivity of the current system and broaden the UV-plane coverage. This
advancement would enable the system to achieve competitive sensitivity with the
current generation of long-baseline optical interferometers over blue
wavelengths.
