Help, sound setting

[iSilent23]

Hi just signed up...I just got an s9 for the first time after years with iphone, I need help understanding sound settinfa

So first small problem is when on YouTube or watching back to the Future ( only movie on my phone ) any notification that comes the sound 100,% over laps YouTube and the movies, it does not fade in and out like it did on iPhone no idea what I am doing wrong

2nd when a head phone is plug in, it auto plays my music, then all notification and system sounds comes out of the headphone speakers and phone speakers...why is that? And can I fix it?

Thanks
Silent

 

[pkcable]

You may need to turn on DND or Do Not Disturb mode on your device. It should be available in your swipe down. It looks like a circle slash or stop sign.