A question about streaming?

Some questions about Twitch and OBS Studio

How can I insert music at OBS
How can I make a start screen at OBS where there's a countdown?
How can I make a transition (yes click on slide in OBS but nothing happens there)

How can I insert music at OBS

Huh? You mean let the music run on the stream?
You can activate display audio (so that you also have game sounds) and at the same time the music runs even when you play something.

How can I make a start screen at OBS where there's a countdown?

Just work with a new scene on the far left, you have the scenes. There you can create a new one with "Start", display the picture and the timer.

How can I make a transition (yes click on slide in OBS but nothing happens there)

Just click on the other scene, then it switches. Or do you mean custom transition like Monte and co?
If so: honestly no idea, but there's definitely an option for.

Yes but where can I create a scene like

On the lower left side of the OBS window.

Believe either with a right click or there's n +. I'm not at home right now so I can't look it up.

Yes, but then what I've done before, but how then how do I create it?