Why is there a $1.5 billion buy order sitting at 0.0001?
Because someone put an order in for it, just like everything else on the book
then why isn’t the ratio "% of vol shorted" stays constant,
Because not every trade is done like that.
Why isn't the market maker match the buy/sell orders from what is already sitting in the book first?
There are no matchable trades, that is why there is a bid and offer. If put a buy order in at the offer it is filled and the inverse as well.
Why is the market marker has no buy/sell orders already sitting in the book when there are 1.5 billion buys reported online ?
That is someone offering to buy 1.5 billion, it will sit there till someone wants to sell to them it is not a transaction. You don't seem to understand what the level2 information is.