As a war refugee, you are entitled to get support from the German state. In the emergency shelters you will receive this in the form of benefits in kind. Later on, you will receive a monthly amount that will cover your basic needs.
If you have taken Ukrainian money with you, it sadly might be difficult or even impossible to exchange it into euros at German banks. The European Union says it’s working on it.
Of course, you are free to use your Ukrainian credit card – it should be working at most places. Same applies to paying via Smartphones.
Therefore, it is better to withdraw euros from your bank account in Germany. Many banks will be happy to help you with this.
You should also be able to open a German bank account.