Enabling Audit Logs in SharePoint Online

You can enable audit logs at Site Collection level in SharePoint Online.

Go to Site Settings >> Site Collection Administration >> Site collection audit settings

Automatically trim the audit log for this site is Yes by default and grayed out.

You can choose the number of days between 1-90.

Path of the default Maintenance Log Library. [_catalogs/MaintenanceLogs]

https://spdevonline.sharepoint.com/_catalogs/MaintenanceLogs/Forms/AllItems.aspx