In recent years, a technology developed between the 1950s and 1970s by the pioneering experiments of Hanbury Brown and Twiss has (re)emerged: the Stellar Intensity Interferometry (SII). Unlike amplitude interferometry, SII exploits a second-order effect that can be measured by correlating the light intensities at different detectors without the need to make the photons interfere directly. This means that telescopes do not need astronomical precision in their reflectors, but only for having several electrically interconnected telescopes with large collecting areas coupled with ultra-fast photodetectors and electronics - the main characteristics of Imaging Atmospheric Cherenkov Telescopes (IACTs, i.e. facilities for gamma-ray observations). The implementation of this innovative technique in existing IACTs provides the basis for current and future facilities to perform optical imaging on the microarcsecond scale. MAGIC is a system of two IACTs located on the island of La Palma (Canary Islands, Spain) designed for the observation of Very-High-Energy (VHE) gamma-ray sources with energies from a few tens of GeV up to tens of TeV. The two telescopes are equipped with 17 m diameter parabolic reflectors and fast photomultiplier (PMT) Cherenkov-imaging cameras. In 2019, the first interferometric measurements were successfully carried out using the two MAGIC telescopes, demonstrating the feasibility of performing these types of measurements and the potential of the MAGIC-SII system. MAGIC-SII has now been upgraded with a real-time, dead-time-free, 4-channel, GPU-based correlator. The upgrade allowed to make the first measurements of stellar diameters with MAGIC-SII. 22 stellar diameters were measured, 9 corresponding to reference stars with known diameters for validation purposes and 13 without previous measurements. All measured diameters were in good agreement with previous observations and/or theoretical expectations, with a sensitivity an order of magnitude better than previous generation of SII instruments. We will show how these results have been achieved and what are the current and future performances of the system. The first prototype of the large diameter telescope (LST-1) of the forthcoming Cherenkov Telescope Array (CTA) will soon be integrated into the system. The addition of LST-1 to MAGIC-SII would significantly improve the sensitivity of the current system and extend the coverage of the UV plane, paving the way for imaging. This advance would also enable the system to achieve sensitivity approaching that of the current generation of long baseline optical interferometers over blue wavelengths.