Go to medicare.gov for info - and like all Gummint stuff, it's almost mind-boggling at first.
There is an enrollment period every year, and that period is also when you can make changes if needed.
There are people who know the system, but they're not likely to be the ones calling on your phone and speaking with an accent.
Some hospitals and providers may not accept some insurers, like Humana - check with them prior to adding non-Medicare supplements.

In my case, with Medicare & the Plan G from MoH, I paid nothing for detached retina surgeries, cataract surgery, or hip replacement, except for medications.
I didn't add coverage for meds but could at approx $150/year.
Being blessed with generally very good health, that's been basically a "wash" so far.

I was on an employer plan, and dropped it at 65 to go Medicare. I work for a very small company, so the size of the company may have something to do with it, I don't know. I would investigate further to see if you can go off the employer plan at 65. It get's complicated but talk to a Medicare expert for advise.

My parents also have an advantage plan. I don't remember which provider. Their plan covers the cost of part B, they pay nothing. Both are extremely happy with what they have. Knee replacement, back surgery (multiple) and my dads biggest bill was $800.

It is a bit of a scam.

Hard to plan for because they change it every other year is seems , Advantage is mostly useless for me , not enough population and not enough options due to that.

I'll be 65 in 3 1/2 years and it's going to help financially to get on medicare. Right now, just coverage for myself (ex-wife's insurance covers the kids) is $1,400 and it's sure to go up every year. I figure if I only have to supplement $300 a month it's still a whopping savings.

It depends on the number of employees on the plan. My wife is covered under my health insurance plan at work, it's a family plan. The factor for her having to start the Medicare was because we have less than 20 people on the plan at my company. I am pretty sure more than 20 would eliminate the penalty like you said.

My wife works for a very large company.

There are 10s of thousands of employees worldwide
I think there are over 10,000 employees here in Tucson alone.

IMO,
if one looks at the costs and coverages the choice should be easier to make.
The first year I was told in error that I was not eligible for the previously favored Plan F. I can't remember which plan I signed up for but there was an additional 200.00 + monthly deduction.
After looking at the options more closely, the following year I signed up for a no cost Advantage plan through Aetna with a max out of pocket cost of 4500.00 with a lot of items being covered at 100% including allowances for dental, hearing aids, glasses and a lot of tier 1 & 2 drugs.
Saving the 200.00 a month for 2 years would more than pay the max out of pocket cost plus the allowances for other things mentioned.
When one looks at the additional savings over the 2 year period factoring in the allowances it becomes even more enticing.
This year I have encountered a few issues with the allowances and am addressing those with Aetna. They have been 100% up to this point.
It can be mind numbing. The best advice is to start learning about the pro's and cons of each choice as far in advance as possible. DO NOT try to learn about them all at once but rather a little at a time, absorb the info, then move forward.
When you start narrowing down your choices check the google and online reviews for the plans you are considering.
One company I will not consider under any circumstance is Blue Cross Blue Shield. Why? they changed a contracted Obamacare plan mid stream due to a peeing contest with a large hospital / firm which left 1000's (myself included) not being able to see their normal Dr.s
Hope this helped a bit

Anybody under 65 could look at something called Medishare if they like living dangerously, but want to save some serious bucks on coverage. It’s religious based, and they can decide to not cover your bills if it’s a “sin” thing that is doing you in. Lots of people like them, but they got banned in Montana for not covering stuff. It’s not technically insurance. People vote on if they agree to “share” the cost of your care. Been around for a decade or more.

This is timely as I retired effective 1 July 2023; State of Alabama, Department of Labor. At the time I was covered completely through Blue Cross Blue Shield - Medical, Prescription, Dental, and Vision for less than $100 per month as I had over 28 years of service, which accounted for the low monthly cost.

Since I had turned 65 back in June of 2023, those two weeks that I worked past the age of 65 reduced my Insurance cost after retirement significantly, well, at least the Part C and Part D. Through Medicare, Part A has no charge as has been mentioned while Part B is automatically deducted from my SS Direct Deposit and since I can't have one without the other, I do essentially 'pay' for both. In my case $164.90; this now covered Part A and Part B. Here's the fortunate circumstance in my scenario - my BCBS immediately transformed into United Healthcare to cover Part C and Part D and since I worked two weeks past my 65th birthday, that cost dropped from what would have been slightly over $300 per month down to $23 per month and became my coverage for doctor visits, Dental, and Vision. That $23 is automatically deducted from State Retirement Direct Deposit.

A little background as 2023 became a long, strange trip of a year for me beyond simply retiring. Circa Christmas of 2022, I developed symptoms of what would be diagnosed as an enlarged prostate. Long story short, I wound up with a catheter (inserted where you never want one inserted ) for 7 weeks give or take before Thulium laser reduction of the prostate surgery basically rolled the clock back 40 years for me. To paraphrase George Carlin, I'm now able to drink all the beer I want and break up a Kent with a Micronite filter again. Regardless, that's what prompted me to call it an (I T career) effective July first. Then, on November 4, 2023, I got married to a wonderful lady who owns her own home along with 4 1/4 rural acres outside of town; we'd been engaged just shy of two years and met at work. She's more than happy to support my Mopar addiction, and even encouraged me to build any size garage I want on the site of an old, dilapidated barn just off the house proper. The only rule is no yard art.

The key, in a nutshell - at least for me - appears to be that I worked past the magical age of 65.

Hope this helps, and apologies if I got too wordy...