If the A invests $1000 in bitcoin, then the next day the price of bitcoin drops dramatically and the A loses $1000, which makes me curious, where does the $1000 A money go?
I think you asked your question with an inappropriate example.
Let me clear your question with a right example:-
If A invested in bitcoin at price of $1000 that means someone sold his/her bitcoin to person A, cause you can only buy bitcoin or cryptocurrency when someone else sell his/her bitcoin or cryptocurrency.
Money doesn't go any where its just keep moving from one hand to another.
If you buy a piece of bitcoin at $100 and you sell that bitcoin at $1000 that means you profited $900, and that $900 comes from that person who buy your bitcoin at $1000.
The simple trading is totally depends on demand and supply if demand increase then price will go high and if demand decrease then price will go down.