A joined report can contain data from multiple standard or custom report types. You can add report types to a joined report if they have relationships with the same object or objects. For example, if you have a joined report that contains the Opportunities report type, you can add the Cases report type as well because both have a relationship with the Accounts object.

A joined report consists of up to five report blocks, which you add to the report to create multiple views of your data.

Each joined report has a principal report type. By default, the principal type is the first one added to the report. For example, if you create the joined report by selecting the Opportunities report type, and then add the Cases type, the Opportunities type is the principal report type. The principal report type controls how common fields are named. Some common fields have different names or appear in different sections in different report types.

When a joined report contains multiple report types, some fields are identified as common fields. A field is a common field if it’s shared by all report types or if all report types share a lookup relationship to the field. Common fields can be used to group report blocks. In Lightning Experience, common fields are identified by the Common Fields Icon icon. In Salesforce Classic, common fields appear in the Common Fields area in the Fields pane.

For example, check the below link

https://www.infallibletechie.com/2019/08/how-to-create-joined-report-in.html