The firestore database is a document-based noSQL database. This means that the data you want to store is in documents. The documents are organized in collections. The document itself is a set of keyword-value pairs. The value can be a single string value or another set of keyword-value pairs. The document has no schema, meaning there is no restriction to what set of keyword-value pairs are found in the document.
The Firestore database stores what are called documents. Each document is essentially a JSON object with fields and properties. In JThermodynamicsCloud, the documents stored are the dataset catalog objects derived from dcat:ChemConnectThermodynamicsDatabase which ultimately are derived from DCAT ontology's dcat:Catalog.