There's also a difference in US and Europe conventions. In Europe, there's typically a 2-year MS followed by a 3-6 year PhD. In the US, these are typically combined into a single 5-8 year PhD program. So an "all but dissertation" phase in Europe is the entirety of the program, it's only the second (and most important) part in a US program.

Yeah, and so if you skip the masters then day one of your PhD is the start of your dissertation.

In practice in the U.S. the early part of a PhD program can be very similar to a master's. It's relatively common for people who leave PhD programs to walk away with a master's degree they didn't intend to get. It's called "mastering out."

Yes you can also leave with an MPhil in the UK if it goes wrong.

I though dissertation referred to the final written document you had to produce to get your PhD. Most people I know with PhDs stared writing that during the last year of the PhD program.

When you're writing papers during your PhD you're basically writing chapters of your dissertation.