Some questions about the media library

Hello,

I have some basic questions on this subject, such as what is a (media) library? Is that a folder with some audio and / or video files as its content or does it simply contain only shortcuts or links to audio and / or video files stored / existing elsewhere? So, if it's the first one, does that mean that it makes a copy of all the audio and / or video files that are in the library (from where they are originally stored / existing) and places those copies in the library folder?

And the audio and video files that are included in the library, in what folder are stored / saved on the Windows 10 PC, on the iPad mini 4 and on an Android device?

Does each PC application of media manager and mobile application create or have its own library folder or do they all use the same library folder? And if they use the same, is it the same as the operating system they are in (if each operating system has its own standard media library folder)?

Which way is to prefer?

Why do not they just use the same folder in the library?
SEMrush