Instead of every visitor connecting to one origin server, requests go to the CDN's nearest edge server. Cacheable content such as images, scripts and sometimes whole pages is answered from there; the rest is fetched from the origin. The results are lower latency, less load on the origin and, with most providers, absorption of traffic spikes and denial-of-service attacks.
A site is usually put behind a CDN through DNS: either a CNAME from www to a hostname the CDN provides, or by moving the zone to the CDN's nameservers, or with A records pointing at the CDN's anycast addresses. The CDN terminates TLS, so the certificate visitors see is the one installed at, or issued by, the CDN.
This shapes what a domain lookup can show. The IP addresses, ASN and location belong to the CDN, not to the company that actually hosts the site; the origin is deliberately not visible. Response headers often reveal which CDN is in use, and a DNS change on a CDN-fronted site can look different from different places because the CDN answers differently per region by design.
Example
www.example.com. 300 IN CNAME example-com.cdn-provider.example.