Anyone knows a good article/tutorial to build a small drone (Quadcopter for example) controlled by custom app (via wi-fi or Bluetooth for example)

I am totally new on this, and I need somewhere to start.
it is important to control it via custom app

Controlling via app is terrible for direct commands, the only useful thing for those would be gps/autopilot inputs.
Also, most custom-built stuff (control boards and rx/tx modules) is designed to work with the dedicated remote controls.

The Navio and Reach families of boards (by Emlid) have some interesting stuff that you might wanna look into, though. Afaik they do have companion apps that make life pretty simple. Should be a good starting point.

And for the love of god: don't be that asshole that flies in dangerous places and puts people - or worse: aircraft - in danger.
Flying drones legally is complicated enough as it is already.
Your #1 priority is safety: if you don't know what you're doing, or you're not sure if it's safe, then don't do it.
(Nothing against you, but I've seen far too many people doing stupid and reckless things with drones because they were too lazy to stop and think for a few extra seconds before acting).

And keep your fingers out of those props!

Out of curiosity: what's your goal?