I work at a large CDN that also sells DDoS mitigation.
Firstly, we are built to endure any DDoS the internet has yet seen on our peering, backbone, and edge servers for CDN services. This is quite important when you are tasked with running a large percentage of the interweb but probably not practical for most organizations, mostly due to talent rather than cost (you need people that actually understand networking and systems at the implementation level, not the modern epithet of full stack developer).
But, it is critical to have enough inbound peering/transit to eat the DDoS if you want to mitigate it -- CDNs with a real first party network are well suited for this due to peering ratios.
Secondly, when you participate in internet routing decisions through BGP, you begin to have options for curtailing attacks. The most basic reaction would be manually null routing IPs for DoS, but that obviously doesn't scale to DDoS. So we have scrubbers that passively look for collective attack patterns hanging on the side of our core, and act upon that. Attack profiles and defense are confirmed by a human in our 24/7 operations center, because a false positive would be worse than a false negative.
Using BGP, we can also become responsible for other companies' IP space and tunnel a cleaned feed back to them, so the mitigation can complement or be used in lieu of first party CDN service.
In summary, the options are pretty limited: 1) Offload the task to some kind of service provider 2) Use a network provider with scrubbing 3) you've hired a team to build this because you are a major internet infrastructure.