Super-Earths and sub-Neptunes are the most abundant exoplanets discovered in our galaxy to date. However, much of their nature and origin remain shrouded in mystery. Broadly speaking, super-Earths and sub-Neptunes are thought to have formed as a single population with primordial hydrogen-dominated envelopes. However, most super-Earths subsequently lost their primordial atmospheres through thermally driven winds. In my talk, I will present new planet formation and evolution models that incorporate the interplay between physics and chemistry and apply them to Earth, super-Earths, and sub-Neptunes. I will show that magma ocean–atmosphere interactions expected in sub-Neptune exoplanets produce signatures in their transmission spectra that are readily observable with JWST. In addition, hydrogen is efficiently sequestered into the interior, oxidizing iron and endogenously producing water. I will conclude by discussing possible parallels between Earth's formation and that of super-Earths, shedding new light on Earth's primary water reservoir, the origin of the light elements in its iron core, and its oxidation state.