If I am a company that builds technical solutions, and I sell it to someone. Then, that someone loses money because the solution did something it wasn't supposed to: who's responsible?

Me as the person who sold it? The vendor of a core library I use? AWS who hosts it? Is my customer responsible themselves?

These are questions that classic contracts typically cover and the legal system is used to dealing with, because technical solutions have always had bugs and do unexpected things from time to time.

If your technical solution is inherently unreliable due to the nature of the problem it's solving (because it's an antivirus or firewall which tries its best to detect and stop malicious behavior but can't stop everything, because it's a DDoS protection service which can stop DDoS attacks up to a certain magnitude, because it's providing satellite Internet connectivity and your satellite network doesn't have perfect coverage, or because it uses a language model which by its nature can behave in unintended ways), then there will be language in the contract which clearly defines what you guarantee and what you do not guarantee.