It explains about upsert statement under DML with Record Id and Custom External Id:
References:
https://trailhead.salesforce.com/content/learn/modules/apex_database/apex_database_dml
https://trailhead.salesforce.com/en/content/learn/modules/apex_database/apex_database_dml
https://developer.salesforce.com/docs/atlas.en-us.234.0.apexcode.meta/apexcode/langCon_apex_dml_section.htm
