Before Citymapper existed, there was OneBusAway, a Ph.D. student project at the University of Washington.
It still exists and powers millions of transit rider trips every day all around the world in Seattle, Washington DC, New York City, Poznan Poland, Buenos Aires Argentina, Adelaide Australia, and who knows where else.
If you’re interested in hacking on something like Citymapper, or setting up an OBA server for your own city, you can find everything you need on our GitHub organization: https://github.com/OneBusAway
That includes docker images, an iOS app and a trip planner framework, android app, Sveltekit web app, and even a next generation OBA server written in Go.
As far as the data to power this, you can get GTFS for every US transit agency from https://mobilitydatabase.org/
(nb I’ve been involved in the OBA project since 2012)