An aircraft’s STCs page lists every supplemental type certificate the FAA has issued for its make and model, or against its type certificate (14 CFR 21.113). The line under the title gives the count and the date the FAA’s list was taken. Each STC number opens the certificate on the FAA’s Dynamic Regulatory System.
The search box takes a holder, a word of the title, or an STC number. The list says what has been approved for the type. What is installed on the aircraft is a matter of its records: the FAA Form 337s and the maintenance record entries.
An amateur-built type usually has none, and the page then reads “None on the FAA’s list for this model.”