Hi Pascal,
I totally agree this is important to understand, to be mentally prepared for the real exam.
My interpretation of the reason why they call it a "built-in" break is that it is not up to the candidate to decide when to take a break, but it is somewhat pre-arranged. As you correctly reported, PMI and PearsonVUE refer to the break as starting between 85-100 questions; in my case, it was after the first 89 questions.
1. The 240-mins exam duration excludes the 10-min break, which is extra time.
2. There is no specific duration for each of the two parts: the countdown timer pauses whenever you enter the break, and restarts (from where you left) either at the end of the break or when you stop the break—as 10 minutes is the maximum allowed duration for the break, but if you want to go for the second part of the exam before the 10 minutes pass you can just stop the break and continue with the exam.
A 10-min countdown timer appears on the screen when the break begins, so you always know how much time you have left. Also, the audio/video recording remains active for the entire break.
Please, notice that the time you spend reviewing any marked questions (in each of the two sections) is still part of the 240 mins; the timer pauses only when you submit the questions, after you reviewed them.
You also have the option to skip the break—as you mention, but this will not affect the exam being split into two parts. You will need to submit the first set of questions, then you are offered the opportunity to either start or skip the break, and then you will go for the next set of questions (with or without the break).
Hope this answers your questions.
Gabriella