Let me start answering your multi-part question by breaking it down. For starters, “fortified” wines are just that. They’ve had alcohol (usually in the form of neutral grape spirits (brandy without the oak aging) added to them. Whether it’s a must being fortified (in the case of port), or a finished low-alcohol wine being fortified (as in the case of some other specialty beverages), the result is still a “fortified” wine.
And yes, you are right in observing that the fortification can happen at different stages in the winemaking process. This is one of the many variables that winemakers can alter to affect a different outcome in the final product. Ports are red wine fermentations (musts) that are allowed to ferment down to about 6 percent alcohol before they are fortified to arrest the fermentation. The fortified must is then pressed and the product is stable due to its high alcohol content. Other fortified wines are just simply products of a normal gone-to-dry fermentation that has had alcohol added to it to boost the ethanol percentage either for stability (in the case of wines to which sugar has been added as well) or for alcohol’s own sake.
For home winemakers to make a fortified wine with some residual sugar, they simply must fortify (add brandy or other spirits) the fermenting juice and arrest the fermentation (by killing off the yeast with the high alcohol content) while the desired amount of sugar still remains in the fermenter.
For Port-style wines from kits/concentrates, a winemaker would perhaps not dilute the concentrate as much (maybe leaving it at about 25 percent to 30 percent sugar as opposed to the usual 22 percent to 24 percent), then would inoculate with a commercially available sugar-tolerant yeast, and then fortify to stop the fermentation when the sugar was down to about 6 percent, according to taste as always.
There are many recipes and techniques on the World Wide Web for things such as this, as well as in many home winemaking books. I hope my comments have clarified some of your questions- happy fortified winemaking!