Well the way laptops and anything else generally works is that the power goes to the battery, which then provides power to the device, in case something is unplugged.
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That is not true, unless you buy those 400$ laptop, or get screwed over. High-end laptop (and pretty much all business class system since a very long time ago, as the market is willing to cash out more for smarter engineering through the system, unlike the consumer that only seeks for the highest numbers on the specs, and lowest possible). When plugged in, the system runs on the power plug. In some systems, its more obvious, by hearing minor interferences when using headphone, due the ground is used by the power adapter, and commonly shared by the system ground.
In addition, some systems won't charge the battery past ~95%. Meaning you'll see it goes to 100%, but as soon as you'll unplug it, it will show a percentage between 99% and ~95%. You get only 100% charge, if the battery is charging from bellow ~95%. (Note: The percentage value, I put "~95%" as it varies between manufactures)
This reduces the strain on the battery if you plug and unplug a moment later.