Smaller airplanes work around this by having a "massive spar" with no joints that goes entirely through the fuselage.
In the case of larger planes a "center wing box" can be used, like the newer C-130 serial numbers that are not affected.
Source: commercially-rated airplane pilot who's familiar with military jet design.
Every Hercules has a centre wing box, not just the new ones, and the "rainbow fitting" is the part that attaches the centre wing box to the outer wing boxes. There is a rainbow fitting on the upper and lower surfaces of the box.
It looks like in this case the problem affects aircraft with more than 15000 equivalent baseline hours on their CW rainbow fittings, which is what's being inspected.
Source: am aerospace engineer with Hercules experience
 the rainbow fitting is a curved surface designed to transmit loads between two mating parts. It's called out as 99A in this pic: https://ibb.co/tbhcdS9
Here's a structural analysis of the C-130H wing design:
It's a bit misleading to say they've been grounded; typically what you'd issue is a special/out-of-sequence inspection to be carried out before next flight, which I suppose to someone outside the industry amounts to the same thing but there's semantic difference.
These aircraft are being inspected to ensure they are airworthy and conform to the approved type design; they're not really being "grounded" which would imply they're not airworthy or not compliant with the type design (which is the case for e.g. the MAX 8).
One is less than a hundred. Title doesn't explicitly state that all those aircraft had cracking, but still seems a bit misleading in a clickbait sort of way.
I would think that's a minimum standard, not a pretty high one. Having wings fall off is embarrassing, expensive, deadly, and nobody wants to testify before Congress as to why they let that happen.