Imagine everyone gets 1000 USD loan, freshly printed USD from the government. Money supply just increased by 300billion (assuming it's for US citizens). Does it mean inflation follows? Probably not because people know they have to pay the money back. The price of assets people really need/want or those that are undervalued is going to go up though.
I believe the situation is the same with stocks. They were criminally undervalued for decades (imo still are somewhat undervalued) so additional money supply causes the prices to increase (it's profitable to buy into great opportunity and pay back cheap loan later) but it's only because the assets were very cheap. If they weren't it wouldn't make sense to buy them no matter how much money is around (as you need to pay it back).
What may cause inflation though are programs when you just hand money to people (not as loans but as handouts) as then you just diluted everyone's else money.
What's your reasoning for concluding that stocks were substantially undervalued?
Because another way to read the situation is that stocks are simply the asset of last resort. In a world with no interest, you can't park money in bonds (or CD's etc.) so you've basically got stocks, real-estate and commodities left as options.
Large-scale asset managers are not going to park billions of dollars under the mattress, so basically it has to go to stocks.
No deep analysis on this one. I just think it's a crazy good deal to be able to buy stock of the biggest corporations which lead humanity technological progress, have huge political influence and even at today profit levels return 3-4% per year (in reality much more as a lot of money is invested in research/new projects which are going to be profitable in the future). I think Apple/Amazon/Alphabet/MSFT etc are going make even more in the future and current price is a steal, especially considering those are highly liquid assets with 0 cost of maintenance.
I believe the situation is the same with stocks. They were criminally undervalued for decades (imo still are somewhat undervalued) so additional money supply causes the prices to increase (it's profitable to buy into great opportunity and pay back cheap loan later) but it's only because the assets were very cheap. If they weren't it wouldn't make sense to buy them no matter how much money is around (as you need to pay it back).
What may cause inflation though are programs when you just hand money to people (not as loans but as handouts) as then you just diluted everyone's else money.