I have 110 euro on my Amazon account which are only activated gift vouchers, so no money from my bank account or anything.
Can I use it to buy an Amazon voucher? If not, what happens when I click buy, will the money be debited from my bank account?
Or can I use it to buy non-Amazon vouchers, i.e. PSN or Google Play cards?
You can use credit to buy anything except Amazon vouchers.
If you want to buy Amazon vouchers, you need a different payment method.
But I can buy vouchers from third-party providers, so like I said Google Play, PSN, H&M etc?
As I said, everything else can be bought with vouchers and credit. Also other vouchers, just not Amazon vouchers.