How to generate effective permission of users for specific sites and lists report?
Perform the following steps to generate the Reports:
Select Sharepoint Online Reports from the available vertical tabs. Then from left pane of application, select External Sharing.
On the selection of "External Users having access for Specific Sites" and "External Users having access for Specific Lists" from the list of available reports, a dialog is displayed as shown below:
 

     
After selecting the desired sites, the resultant data is displayed as shown below:
 

     
The below image shows the final window of the "External Users having access for Specific Lists":