I think I am behind the times on how companies deal with operating systems.
1.) If I upgrade to El Capitan, do I need software keys or something? I don't have any of my original packaging, and don't even know if I have the CD or whatever my OS came on?
2.) If I upgrade to El Capitan, how long will it take? And how much data will it take? (I get my Internet through a hotspot and am afraid I will blow my monthly data-plan.)
3.) If I upgrade to El Capitan, can I still use CCC? Or will I get into licensing issues?
4.) If I ever need a clean install of everything, how would I do that? Truth be told, I am not sure how to even rebuild things with Mountain Lion? Did I get a CD with my laptop when I bought it?
5.) Anything else I need to be worried about? I am used to having a legal copy of the OS on CD and being able to rebuild things as often as you like with no issues. I have no clue how all of this works in the modern world!
1) No, not at all. Zero product key or authentication required.
2) You have to download a 4.5 GB file. It should max take 30 minutes to install on your Mac. No more downloading after this.
3) Licenses will be transferred over. You don't need to re-authenticate. Keep in mind you need CCC 4.x or newer in order to work with El Cap.
4) Since Lion (mid 2011), all reinstallation is exclusively done through the recovery partition. Basically, when you got your computer, a separate partition on your hard drive (which is like probably 8 GB max) was made. If you boot into the recovery partition by holding Option on startup, you will access it. From there, you can erase the main drive, deal with other disk utility things, reinstall the OS, etc. When you will upgrade to El Capitan, this partition will be updated to reflect the new OS, that is, when you reinstall the OS, you will reinstall El Capitan. It's also important to note that if you boot into Internet Recovery by holding Command+Option+R at startup, you will have your system download the recovery partition (temporarily), which allows you to reinstall the original OS the computer came with, in case your drive gets corrupted or something. If you have a Time Machine backup, you can use either to restore back to an earlier version of OS X.
5) As I said, it's even simpler now because your CD is both on your drive and accessible on the cloud

There's really nothing to worry about now.