The crux of your post is the question “are they legal” and the answer AFAIK is “yes”. That’s the easy part. A militia is just a collection of any able-bodied people of legal age who can help provide for the common defense.
The Bloomberg article (Bloomberg is pretty left leaning so take everything on there with a grain of salt) says the following which I think is untrue as a blanket over all militias. Some of them, sure, but far from all. At the end of the article they predictably tie white-supremacists to militias to gun owners to far-right to Trump.
The word “militia” now describes loosely organized groups of civilians who usually share anti-government views and strongly support gun ownership. Some are even preparing for a new Civil War.
It’s a very broad and vague definition. Able bodied can mean a lot of things, and common defense of what or whom? Very likely if you are able to read this, you are technically part of a militia (old enough, and likely with a firearm). Some states have a state organized militia (not always called a militia, in Virginia it is the State Defense Force i think) but most do not. I think Colorado springs to mind (i could be misremembering) as a state with several loosely organized militias. There are probably a few states that don’t have any organized militias.
The danger, as with most things, is in the implementation.
A militia of your neighbors who band together to keep antifa out (several towns/neighborhoods were documented as doing this over the summer) is totally fine. A militia which is actually just an excuse to go camping and shooting with your buddies a few weekends a year as “training” is totally fine.
However a militia that aims to replace local police is not fine. A militia that aims to displace local (or federal) government is not fine. A militia that aims to promote civil disruption or anarchy (think antifa) is not fine.
As noted in the article, militias have gotten a reputation because of a small group of whackos occasionally stirring up trouble. Most militias are really not that serious nor that well organized.
The bad militias are pretty easy to identify. Trust your intuition on that.
If interested in a local/state militia, do your due diligence and see what it is that they stand for. If any flags, and I mean ANY flags, appear you can’t run away fast enough. If they pass that test, scope out a meeting and talk to some actual members and see if what they say they do matches up with what they actually do. A good militia is probably going to scope you out as much as you scope them out because the risk of a CNN reporter sneaking in to do a hit piece is likely really high right now. Which is probably the single best reason to give them a wide berth.