How is RecId field created in Dynamics AX

For more info on the recID generation, check out help contents (F1) of systemSequence class under AOT -> System Documentation -> Classes.
And the table where recIDs are stored is 'SystemSequences' stored again under System Doc -> Tables.
As to the question of when it is actually generated, my suspicion is some time during xRecord.insert() / xRecord.doinsert()