> Or are we to believe that he just founded NeXT with zero idea of what he wanted the company to make

This is literally what happened. I highly recommend the book "Steve Jobs in Exile" about the NeXT years.

Jobs had a vague idea—he wanted to make a computer for higher education—but that's about it. And after leaving Apple he had to promise not to compete with Apple.

The NeXT team traveled around to various universities and asked them what they wanted from a computer. Then they started to build it… but it became something else (vastly more expensive than the requested price point) because Jobs wanted the "perfect" computer.

> the engineers just so happened to make it?

Jobs was a micromanager, though not so much with the Apple II, which was definitely the baby of Woz. However, Jobs was constantly changing his mind and his requirements, much to the consternation of the NeXT engineers, and the indecision of Jobs played a big role in the failure of NeXT.

You may not consider NeXT a failure because it was ultimately acquired by Apple, but at that point Jobs had mostly given up on NeXT and moved his attention to Pixar. In fact Jobs infamously sold all of the Apple stock he got from the sale of NeXT, and the Apple board had to beg a reluctant Jobs to become interim CEO, because they couldn't find anyone else who wanted the job of rescuing a failing company.

To be fair, sales is crucial. For example, Jean-Louis Gassée totally botched the presentation of Be to the Apple board, making no effort, assuming it was already a done deal, whereas Jobs and NeXT nailed it.