For example, from what I understand,
If Alice and Bob share an Bitcoin Account that holds 500 Bitcoin.

Alice could spend all 500 bitcoins on a car in North America when Bob spends it on a different car
in Asia. Both transactions would go to "pool" to be processed. Both of them would be processed in the next block.

What I don't understand is, 1) Would the transaction be visible on explorer site ? Both of them ?

2) I get that maybe a few blocks later, the longest chain would be taken and one of the transactions
would be revoked. But how is this reliable at all? The two cars are probably in possession of Alice and Bob already.

The seller needs to wait at least 4 blocks after the transaction is complete?
Also, regarding the 1st question, what happens on the explorer when the longest chain is taken ?
Before longest chain is picked, which one is displayed?

I am very confused with this. I would really appreciate a reply. Thank you very much in advance.