What is the legality extent of file sharing?

When you buy a book, you buy the rights to read it. Not to own the actual content. Which means you have the rights to own one copy. Again it's COPY. When you only have rights to own that particular copy, you cannot copy those copies.

That is why it is copyrighted. Which means, only the person who own the original material has the right to copy and not you.

But however, you are allowed to make copies for your own personal use (not public, not sharing to anyone). So , in your terms, you can copy what belongs to you, as long as the copied materials remain with you.

If I as an artist produce a record at a cost of $100.000 and you buy it and then share it with everyone then I will lose all my money and will never be able to make a record again.