Tech Ingredients (YouTube creator) made an instructional video about how microwaves work and how to defend against them: "https://www.youtube.com/watch?v=Lg_aUOSLuRo".
No, no it can't. It is physically impossible for any part of cellular infrastructure, be it the phones or the base stations, to direct enough power at an arbitrary person to cause any effect. Phones simply don't have the power to harm a human and the inverse square law means that even high power base stations are safe from more than a few feet.
TLDR; 93/100 available studies (2015) confirm that low-intensity RF causes oxidative-stress mediated tissue damage (incl. DNA damage) and is implicated in a wide range of cancer and non-cancer pathologies.
TLDR2; its not just about power levels.
I'm an IT nerd, not a molecular biologist, so I can't really comment on the paper in detail but a quick skim gives me the impression that whatever effect they're measuring is both subtle and generally not directly linkable to the kind of effects a person would actually notice.
Something that could be used to cause targeted injury would have to be multiple orders of magnitude more powerful. The most effective way to weaponize hacked cellular infrastructure would be accessing the data traveling over it, not a fantastical idea of turning it in to a RF blaster.
If anything your RF components go up in smoke. Modern chips are designed to only work at the desired maximum transmission power, sometimes even less to depend on idle times and such to save costs. But yeah, it's a nice entertaining conspiracy that keeps floating around nevertheless.
Microwaves are not similar nor are cellular radios able to utilize microwave communications.
Microwave towers are specifically built at great height and are utilized for long distance communications.
One cannot turn a cellular device into a microwave, nor can one utilize microwaves to achieve cellular communications.
For FCC that might be the case. CE/RED in Europe goes even as low (or as high, depending on which way you want to see it) as four orders of magnitude.
What they don’t have is the power level to have much effect beyond a few feet.
Not that I'm agreeing with the parent comment - having worked in the microwave comms industry (and a little bit of radar) for about 10 years, the best explanation I've heard for 'Havana Syndrome' is closer to food poisioning than anything to do with RF. I think the US Government has basically walked back any claims that they were actual attacks recently too.
Microwave is just a term for a largish swath of the spectrum that encompasses millimeter wave, and lots of cell phone bands, and frequencies used in point-to-point microwave links, frequencies used by your microwave oven, wifi, etc.