Probing the composition of exoplanet atmospheres reveals the physical processes at play and provides insights into their interior and formation history. Atmospheric temperatures are key in shaping these atmospheres and setting molecular abundances. However, warm exoplanets (Teq < 1000 K) are not well understood. JWST opened a new window into these atmospheres thanks to its broad coverage from red to mid-infrared wavelengths and unprecedented sensitivity. Carbon- and sulphur-bearing species are now detected with high significance and their abundances can be measured. In this talk, I will give new insights on the atmospheres of transiting gas giant exoplanets focusing on the warm Neptune WASP-107b, the warm sub-Saturn HAT-P-12b, and the hot Jupiter WASP-43b that has a warm nightside. These studies highlight the importance of processes that alter thermochemical equilibrium and the role of the atmospheric temperature, provide clues on the presence and composition of clouds, but also leaves unanswered questions. I will also present other projects that I conduct with JWST, from the calibration of the MIRI instrument to the search for unknown asteroids.