If you look at the top of the Data tab you will see two options Data: Raw and Data: Summary. You can use the Data: Summary tab to visualise custom summary tables of layers in a GeoPackage. The summary tables allow you to visualise the results of
summarise operations where summary statistics for a selection of columns are computed for a set of groups. An example, computing the area of croplands in each village (where village is the grouping variable).
Scroll down to the Table Analysis section of the side bar and select Summary Tables.
In the Grouping variable(s) widget select one or more columns in the active layer that describe each group that you wish to compute summary statistics for.
In the Summarising variable(s) widget select one or more columns in the active layer that you wish to compute summary statistics for within each group. For categorical variables the count of observations per-group is returned and for numeric variables the mean and sum are computed.
Click on the Data: Summary tab to view the summary table.