Why we have breakpoints for 400px or 600px, for example?
These breakpoints are verifying the resolution of the device, but now smartphones have resolutions such as Full HD (1920 x 1080 ) or 2k (2560 × 1440). That's why I'm confused. Why do I have to use breakpoints for smartphones with 400px - 600px if now most of them have 1080px - 1440px?
I have a 1080 x 2340 pixel device, but my webpage looks like it's 500px wide
The pixels we define in breakpoints refer to the viewport size of the device, while the pixels defined in a device's screen resolution refer to the number of physical pixels that device has. Viewport size exist to compensate the difference in screen sizes of different devices.
This unit of measurement is sometimes referred to as “device independent pixels” or “CSS pixels”.
For instance, a laptop and a smartphone, both with a screen resolution of 1920x1080. If the screen of the two devices are treated the same way because their screen resolution is the same, the same content (let's say a 1200x1200 image) displayed on both device would look fairly large on the laptop but would look very small on the smartphone because even though they have the same screen resolution, their screen sizes are very different. But with viewport size, the same content can be scaled depending on the device's screen size.
You can read more about this topic in Difference Between Viewport, Screen Resolution, DPR, and PPI for Responsive Web Development.
Hope this information answers your question.