<p>The successful Kepler and TESS missions have discovered thousands of exoplanets and let the community focus on the characterisation of these bodies. One area of research utilises ultra-high-precision photometric and spectroscopic follow-up observations in order to accurately constrain the bulk densities of terrestrial exoplanets. Combining these observables with Bayesian internal structure modelling that uses geological equations of state, we can start to learn about the compositions of planets around main-sequence stars for the first time. Importantly, by studying multi-planet systems we can conduct comparative planetology that can reveal important aspects that challenge our knowledge of planet formation and evolution via the contrastment of the observational and modelling results of a planet against its neighbours.</p> <p>In this talk, I will present observational studies characterising multi-planet systems initially discovered with TESS and followed-up with ultra-high precision photometry from the recently launched CHEOPS satellite and ground-based RV instruments, such as HARPS and HARPS-N. Additionally, I will discuss our Bayesian internal structure and atmospheric escape analyses, and present the results of utilising such models on several key, multi-planet systems observed with CHEOPS, such as TOI-1064 and TOI-561, that are expected to become cornerstones of exoplanet characterisation due to the questions they raise about planet formation, the system multiplicity, or the amenability to atmospheric observations. Important knowledge about these new systems was uncovered via the refined radii, masses, and densities, a combination of precise observations using a new generation of instruments across different techniques, and cutting-edge planetary internal structure modelling. Therefore, utilising these resources we are at the beginning of a new era in characterising terrestrial bodies outside of our Solar System that will be strengthened with JWST.</p>