The banks own the debt and will try to syndicate it out to new investors later. This is standard in an LBO. The banks provide "bridge financing" for the deal while they line up end debt investors. But the banks are ultimately on the hook if the debt market freezes up, as happened here.
ETA: Financing the Merger section of the proxy lays all this out:
https://www.sec.gov/Archives/edgar/data/1418091/000119312522...
Pursuant to a debt commitment letter (which we refer to as the “debt commitment letter”), Morgan Stanley Senior Funding, Inc. and the other financial institutions party thereto committed to provide to Acquisition Sub (which we collectively refer to as the “bank debt financing”):
• a senior secured term loan facility in an aggregate principal amount of $6.5 billion;
• a senior secured revolving facility in an aggregate committed amount of $500.0 million;
• up to $3.0 billion in aggregate principal amount of senior secured bridge commitments (which commitments may be replaced by the proceeds of the issuance of one or more series of senior secured notes (in escrow or otherwise) pursuant to a Rule 144A offering or other private placement, as contemplated by the debt commitment letter); and
• up to $3.0 billion in aggregate principal amount of senior unsecured bridge commitments (which commitments may be replaced by the proceeds of the issuance of one or more series of senior unsecured notes (in escrow or otherwise) pursuant to a Rule 144A offering or other private placement, as contemplated by the debt commitment letter).