Consider one aspect of "stimulus" is that a bunch of people suddenly no longer had to worry about rent. All that money they would have gone to rent instead when to Dogecoin and GME.
When you increase the money supply dramatically, that money is in circulation, and various factors determine its average velocity. There's also a multiplier effect in the form of debt created on top of base money. But the key point is that almost all money is continuously flowing through an economy. Incidentally, disincentivizing people from leaving money at rest for too long is precisely why it's important for a currency to be inflationary.
It isn’t at rest when it is in bank accounts, either. A large percentage of that is loaned out.
Money is created when people take out loans right? (like for a home or car, or when a buisness takes out a loan for capex)
So if the population contracts (i.e. the boomers start dying) doesn't that mean there should a contraction of money supply since fewer people will exist to take out loans?
No they are not orthogonal. You seem to still not understand. What happens to the money used to buy an unproductive speculative asset? The answer is it keeps moving and, in fact, on average it probably continues to move very quickly so long as it's stuck in speculation limbo. Hence, on average, it doesn't stay trapped in speculation limbo for very long. It's a brief detour, not a destination.
And would you say it's a good thing for the circulation you describe to float free of productive activity? I would say, sure, for a little while and in moderate amounts. But if allowed to expand without bound and continue indefinitely it risks untethering minds and markets from the material foundations that make it all possible in the first place