This website says that the six default breakpoints are XSmall (less than 576px), Small (≥576px), Medium (≥768px), Large (≥992px), XLarge (≥1200px), XXL (≥1400px). It says "each breakpoint was chosen to comfortably hold containers whose widths are multiples of 12."
A developer chooses which breakpoints to use based on how they want their content to look on different devices. One of the main tips when it comes to designing using breakpoints is to design the smallest view first (Mobile first) and then design the larger ones. This website gives some tips for better responsive design practices and says that designing the smallest view first helps you prioritize your content rather than having to shrink it down.
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