Only in the secondary market. In the primary market they require different types of money for settlement.

If you buy an AI issue, then the AI company has the bank deposit and the bank still has the matching reserves needed to buy government bonds in the primary market. All that changes is the ownership tag on the bank deposit.

"Money flows through markets, not into markets."

That said, there is a meaningful difference in terms of who has control of the money, and what they choose to do with it. Bank reserves are a red-herring; before the investor chose to buy either AI or government bonds with it, it was sitting in a bank deposit where the bank had parked it in short-term Treasuries. But prices are set on the margin (because again, money flows through markets, not into them), and so it is the act of that investor choosing to buy AI company bonds rather than government bonds that sets the relevant interest rates of both.

A related confusion is that the bank reserves are parked in short-term T-bills, whose interest rate is largely controlled by the Fed, while the investments we're talking about are AI corporate bonds vs. long-term government bonds. These are three different asset classes that trade on three different markets with three different interest rates.

But looking one step back, the investor might need to sell something else to raise cash to buy the stock?

I suppose in that respect, all investments compete.