There is a kind of captive nut or "threaded insert" welded to the wheel arch and the bolt screws into it. It's not just a Fiat thing.. it's a fairly common way to attach a seat belt to the car.
The bolt shouldn't have just sheared off though.. were you trying to unbolt it for some reason? The only real issue with them is that they're usually longer than they need to be and stick out through the insert, into the wheel arch. The exposed threads eventually rust and then they're particularly difficult to remove, since they're a fine pitch and the rust and crud jams the threads inside the insert.
You needs a lot of release spray and letting that soak in before you attempt to remove rusted fine pitch bolts. If possible, also clean all the exposed threads with a wire brush or similar before you start. If the bolt is nowhere near anything that will melt, heating it with a blow-lamp and then squirting it with release spray can also "shock" any corrosion.. although be careful to let the screw cool just a bit before you spray it.. I set fire to my sister's Murano being a bit careless. It didn't burn for long though... (the spray, not the Murano)
If the bolts are particularly bad and you can get Her Majesty in there, cut off the exposed threaded ends of the bolt.. since dragging them through the threaded insert is hard work, even if th bolt doesn't shear off. The bolt inside the insert is usually fairly clean.
To remove a sheared bolt, you have a couple of options. If there's a lot of threaded bolt shank poking into the wheel arch you can get a stud extractor "socket" on it;
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These are filled with vertical rollers that "jam" against the stud as you unscrew it, so allowing it to turn. Yo'll have to "unscrew" the thread (acw) since you're screwing it further "in" if you look at it from the bolt head (inside the car) side.
If you don't have enough thread to grab hold of, then I would cut the threaded shank off at the insert (flush) and then drill it. You don't need an easi-out etc.. or a reversing drill, since drilling it (clockwise) will tend to unscrew it into the car.
If the worst comes to the worst and you ruin the thread inside the insert, then you can use a (stainless bolt) that's long enough to pass through the damaged insert and sticks out enough to put a nylock nut on the end (also stainless).
Ralf S.