Nebius Group N.V. (NASDAQ:NBIS) rallied more than 18% on July 21 after NVIDIA Corporation (NASDAQ:NVDA) disclosed a 9.3% beneficial ownership stake in the AI cloud infrastructure company. The disclosure, stemming from Nvidia's previously announced $2 billion strategic investment, reinforced investor confidence that Nebius is emerging as one of Nvidia's most important AI infrastructure partners. Why Nvidia's Investment MattersNvidia's 9.3% stake suggests it sees Nebius as a strategically important AI infrastructure partner. The two companies have agreed to collaborate on building next-generation AI cloud infrastructure. Being one of Nvidia's strategic partners, Nebius could also gain earlier access to next-generation GPUs and benefit from closer engineering collaboration.