Hi @Donna Wong,
1.Yes, workflow events can be retrieved in SPO platform through the Audit Log Report. The workflow here means SharePoint 2013 workflow (not Power Automate flow) such as OOTB approval workflow, workflows designed by SharePoint designer.
2.Custom events are those events developed and deployed to your site.
Reference: SPAuditEntryCollection class.
3.The report taking a long time to generate could be due to a number of factors, such as the amount of data being processed or the complexity of the report. If you generate the report for several times but they all take a long time, it is impossible that there is no data in the report.
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