Hi @Noyon Chandra Das ,
1.You can find All Unused Sites based on Last Modified Date. If you don't need these sites, you can delete them to free up space.
The modern SharePoint Online Admin center gives a neat view of all sites in the tenant, along with the “Last Activity (UTC)” column that provides a date with value when List Items or Files viewed/edited/shared in the site.
2.You can use PS scripts to generate all files for a specific SharePoint site that have not been modified in a period of time, for example, two years.
Please refer to this thread: https://learn.microsoft.com/en-us/answers/questions/1339500/generate-all-files-not-modified-since-2-years-in-a
If the answer is helpful, please click "Accept Answer" and kindly upvote it. If you have extra questions about this answer, please click "Comment".
Note: Please follow the steps in our documentation to enable e-mail notifications if you want to receive the related email notification for this thread.