Where can I buy it? Never seen it in my life

Well, in Austria I guess.

Found this:

https://www.traubisoda.at/

I used to live in Hessen for a while and had an äppelwoi phase there. It had a proletarian vibe I liked a lot.

Is it sweet? Is it purchasable all year round in all of Germany?

Yes it is. It tastes a lot like Federweisser as far as I can remember it but with apple flavor.

In hessen you can buy it everywhere. You should be able to find it in bigger non-discounter supermarkets.

Is Apfelwein the same as Cider

Possibly. But I've never drank cider so I'm not sure.

Are these alcoholic drinks, btw?

Cider does contain some. The point of making cider is to get alcohol.

Yes, of course

Then ignore Traubisoda. It's a soft drink.