OK ... here we go ...
There are TWO basic different types of shutters.
1.) Traditionally there were "LEAF" shutters that opened (completely) and allowed the film to be (completely) exposed. All cameras had them, (especially professionals).
Note that "leaf" shutters can (flash) sync at ANY shutter-speed (SS), and thus why professionals always preferred them.
2.) With Interchangeable-Lens-Cameras (ILC's), it became necessary to have a Focal-Plane shutter that (above a certain "sync" speed), exposed the film/sensor by a traveling-slit across the film. Thus, above a certain "sync" speed the film/sensor is NEVER "COMPLETELY" exposed.
But the flash-duration can be as short as 1/50,000s, and thus MUST be fired ONLY when the film/sensor is indeed "COMPLETELY" exposed.
Today that is only at shutter-speeds < 1/160-250s ... and that is the problem (in SUN-light).
In bright-SUNlight, @ 1/250s, a correct exposure requires an f/stop of f/11, (or even f/16 if the SS is only 1/160s and/or you are at a beach/sand).
The problem is that @ f/16, the flash range is only 3-4'
The solution is a "LEAF" shutter that can indeed flash-sync at ANY shutter speed.
Thus you can actually use 1/4000s @ f/2.8 in bright-sunlight, and f/2.8 does indeed allow flash up to 20'.
Note that you can even go a step further and shoot in bright-sunlight with 1/4000s @ f/16. This allows the closer (3-4') to be correctly exposed --- however --- the background will be "black", thus high-lighting (only) the main-subject.
In the coming months, you will hear a lot about "global" shutters ... it is indeed a big-deal (the holy-grail), because it also allows fill-flash at ANY shutter-speed, (for longer-range in SUN-light).
They are a new-type of (quiet) "electronic" shutter that indeed allows for the "complete" exposure of the entire-frame -- as "leaf" shutters also do, (existing electronic shutters do not, and indeed were even slower than Focal-Plane shutters).
Wow ... not having ever learned any technical aspects about photography, this is beyond my comprehension. I am not having a clue what you are talking about :-D
What does the ' sign mean in
the flash range is only 3-4'? (I am not a native English speaker.)
It means 3-4 FEET ... (
1-meter) ... fill-flash range in SUN-light.
Note that by sunlight fill-flash I mean a situation where where the sun is behind someone, or they are wearing a hat and their face is shadowed (dark).
A leaf-shutter camera (bridge like FZ1000-II or RX10-IV) can allow sunlight fill-flash up to
7 meters, (20 FEET).

WITHOUT fill-flash

WITH FILL-FLASH

Notice how DARK it is UNDER-ROOF

Notice the CAR under-roof is now MORE VISIBLE
Thanks for your explanation. The flash used is the in-built flash?
Yes, that is with a low-power built-in strobe.
BTW ... ignore everything that Guy wrote ... he was referring to HSS which is a totally different subject and not applicable to any of the cameras you are considering.
It was a kludge-technology to allow "using" flash at higher shutter-speeds,
but does not give any additional range, and has only a couple of very-limited applications. (It would NOT ALLOW the photos above shot with FZ1000.)
It will soon be totally extinct as the (new) global-shutters have eliminated their (kludge) need.
The ONLY thing you need to remember is that NO camera with interchangeable lens allows SUN-light fill-flash beyond 1-meter, (maybe 2 in some situations).
But "bridge" cameras (with fixed super-zoom lens), do have the "leaf" shutters that do allow greater SUN-light fill-flash. (FZ1000-II & RX10-IV)
And only the FZ1000-II has the FAS screen that allows flip/tilting-out and reversible (for "selfies") for more unique/original shooting positions.
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Since the thread has MAXED out (@ 149) ... I will add a couple of final points here ...
1.) With reference to the "focus/sharpness" post ... resolution-charts can only be trusted in the CENTER of the image, as all LENSES have different focus-planes and thus the top/bottom/edges are a DIFFERENT DISTANCE from the "center". So that explains why there were some contradictory results.
2.) The A7C is a FF, and they indeed would have an inherent resolution & IQ advantage, (especially with a PRIME lens), if that was your ONLY priority.
But I suggest that is not the only consideration as I have stated before, I prefer an image that DOES EXIST over a possibly sharper image that does NOT exist.
So I value the speed & convenience & versatility over ultimate sharpness/IQ, (
that may not even be possible in real-world conditions with typical lenses).
The lenses of the FZ1000 are LEICA, (one of the most famous lens manufacturers in the world).
3.) If you want more information of the FZ1000-II, I suggest you ask of other owners in the "Panasonic Compact Camera Forum" ....
4.) .. Here is another NIGHT shot ... (not taken by me) ...
3.)