Report type is a tool that can be used to pull contact fields into the custom report for Event. Report type defines the set of records and fields available to a report based on the relationships between a primary object and its related objects. To edit the custom Event report type and add fields related via lookup, go to Setup > Report Types and select the Event report type. Then click Edit Layout and drag the fields from the Contact object to the layout.References:https://help.salesforce.com/s/articleView?id=sf.
reports_builder_create_report_type.htm&type=5