It is not "free", it means current shareholders of Nvidia are paying for the remaining money. Their stock is diluted on fresh issue of shares.[1]
The $12B comes from Nvidia the company, the remaining money comes from Nvidia's shareholders directly.
[1] Only if the valuation of ARM is "worth it" the fresh issue of shares will not cost the current shareholders anything. This is rarely the case , if Nvida overvalued(or less likely undervalued) the deal then current shareholders are giving more than they got for it.
The $12B comes from Nvidia the company, the remaining money comes from Nvidia's shareholders directly.
[1] Only if the valuation of ARM is "worth it" the fresh issue of shares will not cost the current shareholders anything. This is rarely the case , if Nvida overvalued(or less likely undervalued) the deal then current shareholders are giving more than they got for it.