The simple answer is that in Collect Earth we are not using the PostGIS functionalities, just plain old PostgreSQL.
The way that data is stored in the database is a bit complex, as the plot data does not follow your standard Relational Database approach. Instead the plot level data is stored using an API called [jOOQ][1] that stores the data as binary chunks, which are basically unreadable to humans. If you export the data to Saiku (*Tools->Start Saiku Analysis*), and you open PGAdmin you will see that a new schema is created in the PostgreSQL DB (called *rdbcollectsaiku*) and this one contains actual human readable data were the coordinates for the center of each plot can be obtained.
The plot location would be in the "plot" table.
![alt text][2]
In the Saiku Analytics tool you will also be able to see the location of each plot by using the Plot - Location (or location) dimension.
In any case, perhaps the easiest solution is to **export the data as a CSV** ( *Tools->Data Import/Export->Export data to CSV* ) file and there you ca find the location_x and location_y (longitude and latitude) for each plot.
Does this answer your question?
[1]: https://www.jooq.org/
[2]: http://www.openforis.org/support/upfiles/pgAdmin3_2019-05-08_13-41-41.png