The WRU states the transaction "safeguarded the jobs of around 150 people", including all players, while also allowing the business to continue to operate - now as a WRU subsidiary.
All match tickets including season tickets together with sponsorships and hospitality will be honoured.
Leighton Davies, chief operating officer of the WRU, and Steve King, WRU chief data and digital officer, have been appointed as directors on the Cardiff board.
"At the time of the transaction, Cardiff Rugby Limited owed the WRU around £9.1m," added Collier-Keywood.
"As part of its commitments in the new draft Professional Rugby Agreement (PRA25) the WRU has committed to reduce the debt generated during the Covid-19 pandemic by replacing it with longer-term capital.
"This amounts to around £3m for Cardiff and this transaction gave us the opportunity to do this at the same time by using £3m of the money we were owed and converting this into an investment into Cardiff, leaving our new subsidiary owing the remaining £6m to the WRU.
"As part of the new PRA25, it is our intention to provide the other three professional clubs with similar debt relief."
The collapse of Cardiff means professional rugby in Wales faces yet another delay to its long-awaited new funding plan.
A deal between the governing body and its four regional sides appeared close to being finalised, although the WRU's rescue of Cardiff has put that initial agreement on hold.
Following confirmation of their takeover of Cardiff, WRU chief executive officer Abi Tierney stated "it is unthinkable for us to allow the demise of professional rugby in the Welsh capital".
Cardiff are ninth in the United Rugby Championship (URC).
Matt Sherratt's side return to action in an all-Welsh URC fixture with Ospreys at Principality Stadium on Saturday, 19 April (15:00 BST).
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