Good question! It's booted from a flash drive to run on a host computer with, potentially, windows already installed.
The Tails OS uses the host computer's random access memory, or RAM, as temporary storage space to run core system services. It doesn't access or modify the hard drive of the computer on which it's running. Remember, with Tails you're booting with a USB flash drive so you're bypassing the host operating system entirely.
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I understand, does it have web browsers and other applications to make usage easier