This only applies to Public Marketplace/ACA/Obamacare plans. You can also get on these plans with a qualifying life event or during Open Enrollment. Public plans can also be canceled at any time. So if you were to find coverage somewhere else that fits you better, there is no issue switching.
Private plans are available year-round! There are no limitations of when you can get on and off of private healthcare.
Yes, there is no contract for any health insurance plan, besides employer coverage. You can cancel at anytime for both the public and private health insurance plans.
ACA/Obamacare is best for those with pre-existing conditions or those needing maternity coverage. It is affordable if you are under the income threshold that qualifies you for government subsidies. If you aren't getting government assistance, it is extremely expensive. These plans are mostly high premium/high deductibles because anyone can get on them.
Most employer plans are great for the employee because the employer is legally responsible to pay 50% of the employee's monthly premium. The downside is that the employer is not responsible for paying anything towards family add-ons. This causes the price to jump up significantly when adding family members and can get pretty costly because the family will be paying full price to be added to the plan.
Private insurance is medically underwritten, meaning you have to qualify based on your health. Since you are in a lower-risk pool of people, you get a lower premium and preferred rates, so they are more affordable than most plans with BETTER coverage.