> How can you be broke as a senior software engineer?

Housing and taxes are both expensive.

They're not that expensive, and if a senior software engineer can't put a roof on top of themselves while paying taxes, how can anyone expect a school teacher or a nurse to do so?

Those are not mutually exclusive. Both software engineers and school teachers can be in the shit. I'd argue that the tax impact is even worse for software engineers (as the top end of their salary is mostly consumed by the higher tax brackets) and their net situation ends up not much above that of a school teacher.

He didn't mention debt. I kinda wanted to be vague, since I'm just recapitulating a breakfast conversation I had nine months ago, and don't know a ton of details. I believe that he had to continue paying U.S. taxes in addition to the higher German taxes.

If your taxes wind up higher, then you have to file your U.S. taxes, but they wind up being minimal to 0, as the U.S. counts the taxes you pay there in lieu of paying to the U.S.. Mostly your extra expense should be in fees paid to one of the tax accountants who actually know their way around these tax laws/treaties.