Each lookup geocodes the address, finds the 2020 Census tract containing it, and reads that tract's row — the same path the API takes for an account holder.
A street address plus either a city and state or a ZIP code. The lookup needs a match at street level or better: a match that resolves only to a city has coordinates in some arbitrary tract, so it returns an error rather than a plausible-looking wrong answer. The geocoder behind this demo covers every U.S. state, but only Georgia is reliably quick — elsewhere a lookup may run out of time and come back as an error on an address it could resolve, which is worth trying again.
FastOpenData