There are two main ways to mitigate DDoS attacks.
1. Those that run parts of the Internet need to make changes to the way they filter traffic. For example, don't allow traffic out of their network when the source IP doesn't belong to the network. Simple things like that would massively reduce DDoS attacks. search for RFC 2827 RFC 5635 for proposed solutions. Your ISP might be able to help here.
2. You use a filtering service that sits in front of your servers and filters out known bad requests. Not such a simple task. You have to pay for the bandwidth too so it is expensive during a large attack. Sometimes more expensive than the cost of downtime! List of service providers https://www.comparitech.com/net-admin/best-ddos-protection-service/Also, the service can not prevent application-level DDoS attacks. Large attacks currently can not really be stopped without infrastructure changes to the Internet.