No, if you hold down the shift key at boot, no maintenance of any kind is going on. At least not any that wouldn't have gone on if you'd booted the system normally. That causes the OS to boot into a kind of lowest common denominator mode to give it the best possible chance of booting. It disables certain functions which can cause the OS to not boot, that's all.
If you use some compressed air, just make sure to shut the system off completely. Sleep mode won't cut it. Shut it off, and since you have a unibody model (I was thinking of MacBook's before which only have 8 screws on the bottom case, MacBook Pro's have 10). It's a simple matter of removing the bottom case then blowing some air into the blades of the fan to break up any dust bunnies starting to form. Just try and keep it pointed at the fans and not other parts of the internals.
And as said, Mac OS X is based on Unix. FreeBSD is the foundation for the core of Mac OS X, known as Darwin. The GUI, known as Aqua, which most people associate with being the OS, is based largely on an OS that was known as NeXTStep. Steve Jobs founded that company after being ousted from Apple in the early 90s, and then when Apple was looking around for companies to buy with an operating system they could use to base the next version of Mac OS on, they ultimately decided to buy Steve Jobs' company NeXT. The also-ran was a little known OS called BeOS, which was significantly ahead of its time. It was doing multithreading back around the late 90s early 2000s, before dual and quad core CPUs were much more than a theoretical concept. Everyone who ever used the OS raved how great it handled multi-tasking. The entire OS, from the ground up, was built around the idea of multithreading, which would have been something truly spectacular to behold if the company could have found the funding needed to keep going. Instead they ended up being bought by Palm in bankruptcy, who never did much of anything with it, and who was later itself bought by HP in bankruptcy. Of course we probably wouldn't have ipads, iphones, ipods, iMacs, etc if Steve Jobs hadn't gone back to Apple.
In any case, under the Aqua GUI of Mac OS X beats the heart of a Unix OS. IIRC, Mac OS X 10.5 was actually certified as an official Unix. The Aqua GUI itself is based pretty heavily on the NeXTStep GUI, which was always known as one of the prettiest of the Unix world.
And a minor bit of useless trivia... Mac OS X isn't even Apple's first attempt at making a Unix OS. A long long time ago, Apple had an OS known as A/UX which came on some old predecessors to the now defunct XServe line. Never really went anywhere, and most people today have no clue they ever existed.