Smart Smartphone Case (Where to begin)

Hi all, this will be the first project I’m attempting. Any and all help will be greatly appreciated.

I’d like to create an external casing for a smartphone that communicates with it using a wired connection. The case would have external buttons that in turn feed data to the phone to process.

For example if i were to have a pushbotton on the case, and when I press it I would like for it to open a specific app or function. What type of hardware is most suited for this? Would the arduino software be able to communicate with the android phone?

Secondly, are there any tips or tricks to hacking parts pulled from old electronics? I have a few parts scavenged that are self contained, but I don’t know how to use them.

For example I have a LCD screen taken from an old digital camera. The screen unit is in good condition, but the bit at the end only connects to its original circuit board. Is there a way around this?

I have some experience with soldering and playing with python, but otherwise I’m a complete noob to this.

Thank you very much for your time.