1) The .38 Spl is on the left and the .380 ACP is on the right. (attached pix) Others have pretty well described the differences. The .38 can be found in usual loadings with bullet weights of 110 gr to 158 gr. Heavier weights can be found up to 200 gr if you look around and one company use to load as light as 95 gr. The .380 usual bullet weights vary from 88 gr to 102 gr. The .38 Spl and the .380 ACP are pretty close to the same bore diameter. The .38 Spl is .357" and the .380 is .355" so neither names truly represent their bullet diameters. Their names go back to their origins and the discrepancies often confuse new shooters.
2) a) MAC-10 could be had in either .45 or 9mm. The MAC-11 was the .380 version.
2) b) The .380 and the 9mm are 2 different rds. The .380 is sometimes called the 9mm Kurz or 9mm Short. It is a lower pressure, lower velocity rd than the 9mm. They are not interchangeable.
3) The .380 has several different names. It's original name was the .380 ACP (Automatic Colt Pistol) which was first chambered for the 1908 Colt Pocket Hammerless. It's sometimes called .380; 9X17; 9mm Short; 9mm Kurz; 9mm Corto, etc. Those are all the same rd, just by different names, and they are interchangeable.
4) Weight of the gun is one way. Gas operated is another, which is rare in a handgun. Locked breech as opposed to blow back operated will result in less felt recoil. Higher grip/lower bore will result in reduced felt recoil. Some years back there was at least 1 company making a recoil reducing guide rod. Never shot one so don't know how well it worked. MagnaPorting will sometimes help reduce felt recoil.
5) It is possible to swap parts to shoot different calibers in the same gun. The old M-3 "Grease Gun" which is normally found in .45 ACP was originally supplied with a 9mm barrel and magazine. To switch one just unscrewed the barrel and replaced with the other. It is somewhat feasible to go from one caliber to another but it is generally easier to stay with conversions where the head case is similiar. There are other factors such as recoil spring, etc that has to be taken into account. A common conversion is to buy a upper receiver to convert a gun to something like .22LR for practice. Trying to stay with the same frame, slide and other parts can be frustrating to get everything working right and reliably. Some years ago H&K sold a model IIRC was called the H&K-4. It was the same frame but with just the swap of a few parts it could shoot .22LR, .25, .32, and .380. Haven't seen one in years. I suspect those who have one aren't selling.
As far as the .357 Mag it is the same bore diameter as the .38 Spl but the .357's case is 1/10" longer. Chamber pressures are considerably higher in the .357. Getting a .357 to work in an auto can tax one's patience. The .357 is a rimmed case like the .38 which can cause feeding problems in an auto. It can, and has, been done but it's not common.
6) a) Hyrda-Shok is a bullet design currently being loaded by Federal. It is an easily recognizeable rd in that it is a HP design and inside the hollow point is a lead post. It's been around quite a few years and started life known as the Scorpion.
6) b) Hydraulic shock is not a bullet but is a reaction a bullet has on a substance. Hydraulic shock can best be demonstrated by taking a milk jug, fill it with water, cap it tightly, then shoot it with a high velocity rd out of a rifle. The milk jug will look like it explodes. Shoot the same milk jug with something like a .22 LR and it will punch a hole in the jug. It results from the bullet passing thru a liquid. It will occur at all speeds altho at slow speeds the impact is very minimal and for firearms is pretty much inconsequential. As speeds increase the amount and result of hydraulic shock increases. Since the human body is largely water the hydraulic shock can play a role in bullet performance if the velocity is high enough. Handgun bullets generally do not produce enough velocity where hydraulic shock plays much, if any, role.
