Q: fusion drive question

You have all the information you need about Fusion Drive in the Apple Support website -> Mac mini (Late 2012 and later), iMac (Late 2012 and later): About Fusion Drive - Apple Support.

As you can see, OS X is installed on the flash storage part of your Fusion Drive along with your most used files, and everything else is located on the hard drive. OS X does this automatically and you do not have to worry about this, because OS X will do the job for you. Apart from that, if the flash storage is full, all your data will be on the hard drive.

That's normal. Fusion Drive makes you see the flash storage and the hard drive as one drive, so that drive you see is in fact both your flash storage and your hard drive.

Do not worry about that. Your file will be put on the hard drive, so you can access to it whenever you want to.

Apart from that, it seems that you are running OS X 10.11.3, but the latest version is OS X 10.11.4. You can use the App Store to update your iMac.

I think I have already answered your question. If I'm wrong, please explain the question better, so I can give you an answer.