I have multiple publications on the subject – how to manage ownerless groups in Microsoft 365:

• Ownerless Microsoft 365 groups deep dive (FAQ)
• Implementing Microsoft 365 ownerless groups policy in large environment
• Microsoft 365 ownerless group policy to send more than 10,000 notifications
• Ownerless group policy configuration failed Failure in configuring ownerless groups policy Please try again
• Manage Microsoft 365 groups membership with PowerShell and Graph API
• Ownerless teams and channels – private and shared channels
• m365 ownerless groups policy email message body format and content tricks
• Feature ID: 180749 Microsoft 365 admin center: Manage ownerless Microsoft 365 groups and teams

And 9 videos :

• Microsoft 365 Ownerless Groups Policy Deep Dive: Introductions
• Policy Overview and Configuration
• Customizing Policy Notification E-mail Template
• How is the m365 Ownerless Groups Policy Looks Like for User
• Scope the Policy Down for Piloting and Phased Implementation
• Configuring Policy for Environment Cleanup
• Configuring Policy for Permanent Use
• Notification Period Ends, Members don’t Respond – What’s Next?
• Reporting on Policy with M365 Audit Log
• Closing notes Policy Tips and Tricks

Here is the Introduction video and content covered in next videos: