Hi Ome,
if you are willing to invest €8,50 into a almost ready to use App you could use the pfod app
pfod stand for Protocol For Operations Discovery
http://forward.com.au/pfod/index.html
The controlling App is €8,50. The pfod-design-App is for free
In the design-App you can define the design of the screen that will be shown when you connect to the arduino
All the sending back and forth between your android-device and the arduino (in this case called a pfod-device) is handled by the libraries
The pfod-design-App creates the arduino-code that is the interface to your own arduino-code.
So pretty much the same as using any kind of library. You connect your Wifi-device to your local WLAN and voila
data can be exchanged between your android-device and your arduino-device.
This app allows only one device beeing connected to the arduino-device at a time.
Another approach might be using the telegram interface described by randon nerd tutorials
usually I ddon't advertise a certain product but this one I make an exception.
If somebody else is aware of a similar product please feel free to post it.
best regards Stefan