I'm certainly not an expert but from what I've read and experienced, even tablets perform a P.O.S.T. (power on self test) when completely shut down and turned back on again. If I'm having trouble with my kindle or phone doing funky things, I'll shut it down, wait about 30 seconds and crank it back up again.
I've met a lot of people who thought that it was turned off just because the screen was off but for most phones and tablets, that's not really the case. Most mobile devices are set to go into sleep mode after 1 or 2 or 3 minutes. It saves a lot of batter power if they do that. But it also makes it very, very fast to "wake up" and use it again.
If you want it off, you must intentionally shut it down by holding down the "wake/sleep" button and the follow the instructions it gives you. Like, for the iPhone, it'll say... "Swipe to power off".
Or, of course, if the battery runs out all the way, it has no other choice then to turn completely off.
But I've also read that's pretty bad for the lifespan of the battery so I try not to ever let the battery capacity down quite so low.