@sogy
A retiree, of any age, can get an a D visa for Bulgaria on the basis of their pension. But if you are already a legal resident it's probably better to do Family Reunification instead (if it's possible).
The standard Bulgarian Health Insurance for Foreigners (as typically purchased to keep immigration happy) is an accident/emergency/repatriation insurance, rather than proper health insurance. So I'd guess you can get a policy at almost any age. And it would be easy to find out. If not, there is real Bulgarian private medical insurance, and I'd guess this would work too. You can get info on both options from your local insurance broker, or online with 24ins or similar.
You'd need to check the Medicare issue separately. My guess is that Medicare and Medicare Advantage is very expensive at 92. And I doubt the coverage extends to someone living abroad (as opposed to visiting for a few weeks). And probably has significant deductibles/exclusions. I very much doubt this would be a viable option, and is likely very expensive if it is. I'd imagine that a Bulgarian private medical insurance would be a better and cheaper way to go.
As mentioned by @janemulberry, I did this recently when I kidnapped my 96-year old father. It was a tough journey, and we only had 3 hours on Ryanair to Plovdiv, with both me and my brother escorting him and trying to keep him distracted. :-) Hopefully, your mom is a bit more robust and could cope with a much longer journey. But bureaucratically it was amazingly easy... and the Plovdiv immigration was outstandingly kind and helpful, it was quite touching. 90-odd is pretty old here so they're pretty impressed that an expat is taking on that move, and they seem very supportive of a family member taking on elder care. So that's part of why I recommend D visa based on Reunification rather than Pension.
I went to immigration first (with a written letter) and explained the situation (that I needed to bring my 96-year old father to live with me in order to care for him). I discussed the options with them, and asked about Family Reunification. I've been here since before Brexit (i.e. quasi EU) and I'm a PR. I don't know your immigration status or the exact rules. It might be possible for a non-EU recent recent resident, or perhaps it's reserved for EU citizens or PRs or Brexiters, and I got lucky. It's also possible that it was an exception/discretionary on the basis of his age/care situation. (If Reunification is not possible, then D visa by Pension it is.)
Medical was also easy, as my dad's a Brit, so we were able to get an EHIC and an S1, both of which are accepted as health cover. As discussed above, I reckon you can get the standard policy for immigration or proper medical insurance. However, when she successfully goes through this process, the 2nd option is by far the better option.
The best option of all is to be covered by NHIF. Again, easy for us because they accept my dad's S1. They won't let "economically inactive" TCNs into the system immediately. Unless they work (employed or self-employed) first, which is probably not a good option for your mom. :-) Or they're PRs (i.e. they've been here legally for 5 years). HOWEVER... if you are a legal resident already (and you do family reunification) then if you are also enrolled in NHIF already, I'd guess this exclusion would be waived. In this case, she would pay the 20 euros per month contribution as unemployed. It's an amazing deal! Especially as you can go to private hospitals too, and only pay the private surcharge, not the full amount. And there are no deductibles, no exclusions, and no age limits. If you're not already in NHIF, you should be!
Moving my dad was not popular with all my siblings. He'd been living in his house for 50 years, so there was some pushback that dragging him away was cruel and inhumane treatment to rip him out of familiar surroundings to a place he'd never been before, and where he would not understand the language. And it was undoubtedly a stressful move that needed a lot of planning and preparation. But it was a fantastic decision, and it has worked out far better than I hoped. I have installed webcams so my sibs (and I) can tune in and see what the old boy is up to. And we have an Amazon Echo so they can do video calls with him. (There's a tablet with Zoom too, but that needs me to be there.) And I have a Google Album that lets them see dad's adventures in Bulgaria. The costs are far lower here than in UK or USA, so that's a huge plus. (We were spending 10k GBP per month!) In particular, we can afford live-in care here, and a geriatric physiotherapist twice a week (so he's way fitter now than when he arrived). His GP and Ramus (blood test folks) both come to the flat when requested.
We're in Plovdiv, so we have big city services nearby. It's probably a bit harder if you're in a village, but I'd expect your local GP to be just as supportive.
Good luck with the mission! And I'm happy to answer questions / discuss my experience of doing this, if you PM me.