He's not rich enough to juice the world economy. We're currently debating spending a trillion dollars on stimulus for the American economy, and many people are concerned that this won't be enough to make a difference.
Engineered systems can have various modes (operational, degraded, maintenance, etc.), bears can hibernate, some frogs can burrow for years in the ground waiting for better times, etc.
Our societies should also be able to switch modes to handle bad times, functioning under different laws until things get better, without all the drama that we see today.
"Injecting" money in the system to try to put it back to "normal" makes me think about a bear taking stimpacks to walk around in the winter looking for non-existent food.
Let's say there is a restaurant on a street corner. They have good food, people enjoy the vibe there and the decor is tasteful. Half a year a go they employed 15 people and the owner made a modest profit.
Now guests have stopped coming. But rent is still due and so are wages.
If somehow political leaders had the time to micro-manage what should they do about this situation? Should it stand empty so it's ready start serving people immediately as soon as corona situation is resolved? Should furniture be put in storage and the space be reused for something else in the mean time? Should the restaurant disappear to make room for something else? The employees should they be on standby? Or take on different roles while they wait?
And then having answered that question we can take the abstractions back, and ask how the outcome can be approximated by manipulating interest rates and helicopter money.