Rational points on modular curves: parameterization and geometric explanations

We show that, conditional on Zywina's effective version of the Serre uniformity conjecture, there is a natural way to parameterize non-CM $\mathbb{Q}$-rational points on all modular curves in terms of the rational points on finitely many modular curves. Our proof refines Zywina's work to give a (conditional) parameterization of the images of adelic Galois representations of elliptic curves. In particular, we show that there are 41 $j$-invariants of elliptic curves whose associated Galois image does not vary in an infinite family. Using our explicit parameterization, we show that all rational points on all modular curves arise from the geometry of modular curves in a formal sense, confirming a philosophy of Mazur and Ogg.