Invantive Cloud is an UniversalSQL-database. This is a virtual database, which real-time interprets SQL statements and translates them in operations on the APIs of the supported platforms. The SQL syntax is documented on Invantive UniversalSQL Grammar 23.0. The Exact Online datamodel is described in SQL-stuurprogramma voor Exact Online API - Exact Online API Data Model.
An Invantive Cloud-database is typically accessed using the OData4 bridge named Bridge Online (see What are the critical points for implementing Power BI? as a good starting page on the most common questions and solutions).
For use with Google Looker Studio it is recommended to copy the data to a SQL Server, PostgreSQL or MySQL database using statements as illustrated in Elementary Data Replication Module between Exact Online and Azure SQL Server.
Alternative approaches are also possible, for instance using an ETL tool accessing the OData4 bridge.