We talked before in previous post ” https://dellenny.com/flag-email-in-outlook-will-add-task-for-you-on-to-do-list/ ” on how to see flagged emails, but in case you did not get notification to allow connection between emails and To Do, you need to go to settings as shown in this blog.
Go to Microsoft To Do and press on settings:
Then on press on To Do Settings this will open this dialog make sure that you select Flagged emails as on as show below
Extra posts:
Allow Forwarding of emails in Office 365 
Listen to your office 365 outlook emails 
Create Rule for emails you are in the BCC 
How to Manage Teams Permissions and Privacy Settings 
Configure Tenant-Level Sharing Settings for SharePoint and OneDrive 
Windows 11 Privacy Settings You Should Change Right Now