When mac is not in use, i turn it off completely, to allow it to cool down properly (as was advised by Apple staff).
I have no explanation for that advice. I disagree with it. If you're not actively using a Mac, ignore it. It won't mind. If you're not going to use it for a prolonged period of time, perhaps a couple of weeks, go ahead and turn it off. It has been my experience Macs work best and last longest if you don't constantly turn them off and on again. Ignoring them in that manner is exactly how they're designed to work.
I am open to any suggestions to how to have my Mac as clean/clear as possible, to get the best experience from my Mac, and to prolong it's use for many years to come.
Macs do not need to be pampered. Use them in the same way you would use any other tool. Macs need about as much maintenance as a hammer, and will last about as long.
The only thing that doesn't come with a new Mac is a backup hard disk drive. If you value the information on your Mac, it needs to be backed up somewhere other than on your Mac. Conversely stated, if you do not back up its contents, by definition you don't care about the information on it. Either way is ok as long as you appreciate the implications of not backing up.
Back up your files with Time Machine on Mac explains how it works. $60 or so will buy a perfectly adequate 2 TB hard disk drive.
The worst things anyone can do to a Mac also happen to be very popular, as a result of aggressive marketing efforts exploiting an understandable desire to keep a Mac "clean" and / or "safe". As a category, those things are scams. Learn to avoid them, just like any other scam.
Macs are tools. So are hammers. Use them responsibly.
