Redirect SP 2010 to SP Online

Hi,

We have migrated large no. of documents from SharePoint 2010 on-prem sites to SharePoint Online sites.

Now we want to let users use old links or new links for which we want to set URL Redirect rules.

For eg:

On-prem Site 1 > Library 1 > files should automatically redirect to

SharePoint Online Confidential site > Library 1 > files

Old URL : http://app.contoso.com/sites/Site1/Library1/filename.pdf

New URL: https://contoso.sharepoint.com/sites/Confidential/Library1/filename.pdf

How can this be achieved?

I have gone through the IIS URL Rewrite Module documentation but it is confusing.

Can someone explain it in easy language specially w.r.t above scenario which we want to implement.

Thanks in advance