Mayo Clinic is a research and teaching hospital. They do a lot of research and do not accept all patients like a regular hospital. The main Mayo clinic is in /Rochester, MN and one is also in Phoenix, Arizona and one in Jacksonville, FL . Some hospitals are affiliated with Mayo which I understand means they have access to some of the research.
Check out US News Report for hospital listings and expertise.
P.S. - People with appointments at Mayo who have had recent Covid, should have all test documentation available. Mayo asks patients to test before many procedures and they do a PCR test which means it will pick up virus for months. They want their tests and not another entity. If you have recent test positive documentation, you can avoid the test requirement. I did.

Why people choose to go to Mayo Clinic? That's a great question, @lenchiksf and you got a wonderful answer from @vic83.

Mayo Clinic's mission to put the patient first is really embedded in its DNA and all the people who work there from the volunteers to the clinicians. You feel like you are the only person who matters when you go there. Another hallmark of the Mayo way is the multidisciplinary approach. Specialists are assigned to you as needed. You are the focus. So if your health situation requires a pulmonologist, a cardiologist, a social worker and a pain management specialist that's who will see you. Another patient may need a completely different combination of specialties. No silos at Mayo.

Here's more info:
- Why choose Mayo Clinic? https://www.mayoclinic.org/why-choose-mayo-clinic

The good news is that more and more facilities are adopting the Mayo way and you can find good coordinated care elsewhere too. Mayo Clinic teaches their approach to care far and wide. In fact, there is something called the Mayo Clinic Care Network (https://www.mayoclinic.org/about-mayo-clinic/care-network) Institutions who are part of the Mayo Clinic Care Network (https://www.mayoclinic.org/about-mayo-clinic/care-network/network-members) are trained by Mayo and have access to Mayo Clinic expertise to treat patients closer to home.