The black fillings are amalgam which is very strong but actually can weaken the tooth, and it’s predominantly made from mercury.
Amalgam is usually replaced with either resin if the cavity is small, or ceramic if more of the tooth is missing. Resin can help the tooth become stronger after it is restored, but if a large amount of tooth is missing, then resin is just not up to the job.
In this situation, a laminate of ceramic and resin is bonded to the remaining tooth to make the tooth very nearly as strong as it was before it ever had a cavity.