Use these steps to attach a file saved on SharePoint Online to an email on the Outlook desktop app.
You can also use SharePoint links, read more here: Attaching Files from SharePoint to Outlook Webmail version
Please see the below video or the proceeding guide
1. Select Attach File
2. Select Browse Web Locations…
3. Select Sites then choose a recent folder.
4. A File Explore window will open up. You can now browse to where the file is saved.
For more information, please visit the Microsoft 365 Resources on the Patriot Knowledge Center.