The pieces that make up your cost, and where the published figures live.
Medicare figures change every plan year, and plan costs vary by county. A number published on an agent's website goes stale quietly and then misleads people. The current official figures are published at Medicare.gov and that is where to get them. What follows is how the pieces fit together, which does not change.
Premiums are the visible number, so that is what people compare. The costs that actually determine what you spend in a bad year are the deductibles, the copays, the out-of-pocket maximum, and whether your specific drugs sit on a good tier. A plan with a lower premium is not automatically cheaper for you.
Both Part B and Part D have late enrollment penalties that are added to your premium permanently once they attach. They are entirely avoidable and they are the most expensive avoidable mistake in Medicare.
If cost is the deciding factor, look at Medicare Savings Programs and Extra Help before you narrow your choices. They change what is affordable.
General information only goes so far. What is actually offered where you live, and whether it fits your doctors and prescriptions, takes about ten minutes to check. No charge for the review.