A good eCig, like an Inokin Coolfire provides:
Constant voltage, boosted as high as 30V.
Constant Wattage, boosted as high as 40W (some vape mods get as high as 100W)
Auto-timeout at 10 seconds to prevent overheating in your pocket if you accidentally press the button.
Everything is controlled via an oLED screen and three buttons.
It also provides USB charging and "pass through vaping", so you can still vape while it's charging.
Constant voltage works by the resistance of the coil and ohms law as to what you get. I prefer using constant wattage. I don't sub-ohm vape, so I'm working on somewhere between 7 and 10 Watts.
There are however reports from some coil vendors that Innokin products have a tendency to spike current before they recover to regulated amount, but I found it was only those manufacturers coils that burnt, so I don't really believe them. I could put my Coolfire through a 2 Ohm resistor and scope it I suppose.
Making your own of similar quality in a small form factor would not be easy to say the least, you need a boost circuit, an 18650 power path charger, a current limiter, a constant voltage limiter, a wattage calculated current limiter, oLED screen and MCU, timeout override circuit etc. In the Coolfire the completely board including oLED is 1cm x 5cm double side populated.