The difference is that money and stocks are 'fungible', whereas a car is not. Something is 'fungible' when it is indistinguishable from its replacement part. If I open a bank account with a $100 bill, and that bank puts it in the safe, then whose money do I get, when I walk around the corner in 5 minutes and withdraw $100 from my ATM?
The answer is that the bill I took from the ATM is indistinguishable from the $100 I used to open the account, so it is irrelevant that the bill is not the same physical piece of fibre.
Likewise, for stocks, if you hold your shares with a broker, and someone shorts that stock, the broker may allow your stock to be leant out [with consent likely part of your ultimate broker agreement, in a simplistic sense]. If you then want to sell your share, technically to sell the same one you would need to recall the short, but in reality the broker will just handle the sale by shifting with another client whose share is deemed to have been leant. And because those shares are fungible, this isn't usually a problem.
In a circumstance where there are no other shares to 'switch', then ultimately the broker bears the risk of not being able to make everyone whole again, which is a risk they bear in order to earn the interest they receive from the short seller when they borrow the share in the first place. You aren't really involved in the process at all.
As to whether short-selling creates additional selling pressure - theoretically yes, the additional short-sale would indicate that yet 1 more person thinks a stock will go down. If the price of a share is largely based on the perception of value based on stock transactions, then continued short selling would further drive the image of that stock's decreasing value. If the price of a share is largely based on the perceived value of the underlying company, then it would be unchanged regardless of which transactions occur. In reality, public perception can have a huge impact on value, and short-selling etc. has a larger impact on resulting share-price for small stock or where short-sellers trade in such massive quantities as to drown out other activity.