The IRS, if it chooses to do so. But auditing large companies with complex governance structures and large legal teams is prohibitively resource intensive for the IRS because it's intentionally underfunded by Congress.
Presumably external auditors would be hired by the stakeholders though. Particularly for the for-profit entity with investors and a lot of money passing through it. I’d imagine it would be conducted by EY, PwC, Deloitte or KPMG.