Explanation
An ID variable is a type of variable that can store an ID value of a record or a record type in a flow. It can be used to store the ID of a record type for later use in a flow, such as assigning it to a record or using it in a condition. A boolean variable is a type of variable that can store a true or false value in a flow. A text variable is a type of variable that can store a text value in a flow. A record variable is a type of variable that can store one or more field values of a record in a flow.
References:https://help.salesforce.com/s/articleView?id=sf.flow_ref_elements_variables.htm&type=5

Explanation
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