TLDR; Car Rental agencies in general are sketchy, especially when booking online, but the people here were helpful (shout out Donation).
This five-star rating is an endorsement of this specific branch, not of Sixt as a whole.
I booked a car online, got the confirmation (success, congratulations, confirmed, this is your car, etc.) but they did not actually have said car. I see this in a lot of reviews! Fortunately they called me right away to let me know (if I didn't have an esim card for my trip I never would've gotten the call and things would've been much worse - if you're from the US get an esim). It's absolutely ridiculous that car rental companies can get away with promising you a car online that is not actually available!
However, at this branch they did everything they could to get us a vehicle. Donation helped us, spoke very good English, and was very kind