In the end with most of these deals its the shareholders paying through market cap dilution. Given the current market structure (the big companies are tech companies) there's PLENTY of capital for OpenAI to fund their expansion.

They've discovered a cheat code IMO. Instead of using and raising money themselves, use their reputation/popularity and use their suppliers market caps (e.g. NVIDIA, AMD, etc). The deal makes sense as long as the value projected to be added (i.e. via efficiency gains, loss of jobs, changing society, etc) exceeds the capital dilution for the supplier; they use their equity but the leftover equity value increase makes up for it.

Given all the passive investing, and funds invested in the top tech companies this is a VERY large pool of capital. It however increases leverage if the value doesn't materialise.