Other platforms such as Windows NT use proprietary software where the code is not openly available to the public to scrutinize for holes. This does not mean that such platforms do not have any holes, only that no one ever finds them. However when they do find them, they don't get broadcast, and you can't go in and fix them yourself once you find out about it. So there is a trade off here. Closed software systems make it harder to find these holes, but also harder to fix them.
Hackers are a VERY large and influential community planet wide in the computer world.
Some are good guys and some are bad guys, often the bad guys are called crackers to distinguish them from the the good guys. The term TO HACK means to put together a new and ingenious solution to a problem. That hacking is sometimes used to gain unauthorized access to systems where they do not belong is incidental. Anyhow most hackers are benign, only a very few of them are out to cause any harm, although a large number of them are definitely into getting into where they don't belong. I imagine somewhere in their minds they have a huge wall map of every system in the known universe, and little pins in all the machines where they have gained root access.