"What's the difference between directly printing money out of thin air and creating bonds out of thin air and selling it for money (that also was printed or electronic)?"

A BOND IS A LOAN, A LEGAL OBLIGATION TO REPAY

With 100's of years of financial regulation and legal apparatus and precedence, long-standing laws, process and practices it.

It's a fundamental part of the credit system.

The price of the bond on the the markets will be a function of the coupon rate, and the 'risk' associated with the issuer actually paying back.

That's why bonds can be resold because they are worth something.

PRINTING MONEY AND SPENDING IT ON MANSIONS (or whatever) IS DILUTION

It's just making 'z dollars in circulation' no 'z*n dollars in circulation' backed by the same amount of assets, ergo, each dollar worth less.

I recommend "Investopedia.com" where you can go and look up what a 'bond' is, what a 'stock' is, what 'currency' is, and what 'central banks' do, it's a great resource.

There is still typically dilution when the central banks buys bonds, only that it's indirect - coming from the increased (overheated?) market value of the mansion(s) used as collateral for the bond.

It's definitely increasing the money supply in that case. If the assets are valuated at the correct price is another issue...

There is a huge class of money that is created when a bank issues an covered bond (backed by real estate), which is bought by a central bank.

The money is both created from thin air and at the same time connected to the underlying assets in the bond.

I'm leaning towards the opinion that it's more "air" than "matter" in that case, but you could argue that it's somehow reflecting the underlying economy somehow.