First, excuse my English, I’m a French guy, who doesn’t speak English very well

Then, I really thank all of you for your advice, it’s really helping me through this project.
My question is about the audio. I’m making this game boy with a raspberry pi 3 and I have the mono amp from adafruit. The following diagram shows approximately my configuration ( Thank Chiz, crispy_tofu, Adafruit). My problem is that I have absolutely no idea about how I can connect the original Jack. I would like that when I plug a headphone, the speaker turns off (yeah, really), and the potentiometer works with the headphones. Thank you !