Hey, my apologies. This is the second time today my visitor's replies are not popping them into my inbox. Thank you for your patience.
OK, so here we go: You will be eligible to Part B if you were married to a spouse who gained enough credits in his lifetime, or you yourself gained enough. If have not had sufficient income to file taxes in the past, and were not required to, then it likely won't matter. Many people who don't work and only collect SS, do not file returns (not that I think that is wise, even if it is legal) and still get Medicare. The typical reason for the need for tax returns IF filed, is so the government can determine if you are eligible for the broadest government subsidy of your premiums, or something less than that. Those that earn over $85000 (MAGI) as a single may be subject to an increased medicare premium for parts B and D, due to "IRMAA" adjustments. Of course, those earning less, which would include those with income of ONLY SS, clearly would not breach that.
Now, the issue of not filing tax returns is a different one, and tax evasion can be a serious issue. But that is only if you have reportable net income from your self employment, after legitimate business expenses. That would more so be a question for your tax preparer to determine. And on that note, I do suggest you contact one. They can be very, very reasonable in price and can draft up a proper return. You'd likely be surprised at how low your "net" income from self employment is after legitimate expenses. I would add that if increased income is needed and you are not finding you can get by on your SE earnings and your SS, it may be time to look at the budget and downsize it (while considering also working as a W2 employee for an employer if that will earn you more) - I say this because for many of us, there will be a time when we are not able to work - and when that happens, and savings is gone, we have to figure out how to live on our SS alone. It can be a good idea to do that long before that reality comes to be.
But going back, Medicare will still be provided for those that don't file returns. They will know your SS income and your premiums would be that of a person making the SS income you are - and if your MAGI is less than $85k, you will have the lowest premium of 104.90/mo.