Three things about the Medicare Supplemental Plans are:
(1) they are not paid for merely by your Medicare premium, you pay additionally (mine $2k plus a year), and \
(2) while they do not have some “free benefits” that some Medicare Advantage plans have (dental, eye care, ect>), they essentially pay certain levels (depending on the insurer, the health care item & the specific plan) above the Medicare 80% amount, and
(3) they usually have no “network” or PPO; and usuually your Medicare Supplemental Plan is good anywhere in the U.S. with any provider that also takes Medicare.
That's what sold us on original Medicare with Medicare supplement. We don't want to worry if the emergency room personnel are in our network.