regarding practices.
The answer I give does not apply to cardiologists in general, but rather to most specialties.
There are very few solo practices around these days. It is very difficult to be available for you patients 24/7. Also it is easier to share administrative costs with a partner.
There are hundreds of variations of private practices. There are small groups of 4 cardiologists; there are larger groups of 20-30. There are groups of interventional only, or EP only. You can be a part of a multi-specialty group where you may get referrals from PCP within that group. You can be employees of a HMO. you can be employees of a hospital, then work your way up the hospital corporate ladder to be the CEO of the hospital.
Almost anything you can think of, exists in the marketplace.
In addition, there are a lot of faculty positions: you can be a hardcore cardiologist-lab researcher, or a clinical researcher, or you can attend inthe CCU. You can be a cardiologist at a community academic center and primilary just focus on educating fellows with some academic projects on the side.
Finally, many cardiologists jump ship after practicing, and work as consultants for law firms, consulting companines, bio-tech companies, governemnt etc.