Medicare Advantage is offered by 保险公司, not standalone thing

Kaiser definitely offers Medicare Advantage. 

Your concept on Medicare Advantage is not quite right.  All Medicare Advantage Part C are offered by Insurance Companies.  They are not standing alone!  Nor administrated by Medicare / SSA.

Someone said they also have Kaiser Medicare Advantage but their additional premium was about $20+.  I replied that we asked our friend about it several years ago WHY his plan so expensive.  He insisted his was Advantage but may be his was a PPO.