The ExoMol database can be navigated in a browser in two ways from the options on the left:

By Molecule: select molecule > isotopologue > data type

By Data Type > molecule > isotopologue

The ExoMol API is described in our open-access article, The ExoMol database: Molecular line lists for exoplanet and other hot atmospheres:

The "master" ExoMol definitions file is ExoMol.all

The "def" files are accessed at URLs of the form www.exomol.com/db/<molecule>/<iso-slug>/<dataset-name>/<iso-slug>__<dataset-name>.def. For example, the def file for the YT10to10 dataset for the 12C1H4 is at http://www.exomol.com/db/CH4/12C-1H4/YT10to10/12C-1H4__YT10to10.def