QWEEDDYZ: Winter Assault is only DLC here. As Expansion (more close to nowadays DLCs is Add-On's included ingame, sometimes thay happened in old days too that games content split to AddOns, Expansion mean add-on with own executable file which is usual thing back in old day). Addon expansion dlc - this is now synonims.
Stand-alone expansion never mean DLC. Dark Crusade and Soulstorm are entitely new games. Soulstorm not include any old content.

This stand alone uses for new games based on old engines with or without minor improvments to game engine while new title in dev (DOW2 in case here). It is like spin-off termin but for entire game and not only for Plot.
So DC and SS new titles spin-off before DoW2. In term of Plot too (DOW2+DLC continue story of DoW1 commander)
DoW2 Retribution was again entirely new title. With dozen addon builtin dlcs. Last Stand stand alone actually a DLC. Game just split to two DLC with campaign amd last stand. You get game+one or another DLC. This stand alone related to digital stores subscribtion packages. In term of technical aspect - it is not stand alone at all. It is installed into base game along with it (same for Shogun 2 Fall of Samurai).

Still previous games required in multiplayer. More like bonus unlocks for having previous games.
Only own game key (DC or SS) required in Multiplayer. Validation kf unlocks keys based on having same keys in Windows Registry where they stored during previous installations. Technically it can be considered as DLC and not. Multiplayer DRM is always separate case and not related to game itself. So those games require GOG Galaxy for online still DRM free, DOW1 games require previous titles in multiplayer still separate titles not related to each others.

Offline content completely separated for all this 3 and one-half games.
Units in SS same as previous but hugely changed balance. Most units available only in 1-2 limit and all units cost too much limit-count. You cant even have them all. DOW1 have better balance for SP, IMO. Maybe SS better for multiplayer. It have two more factions. Maybe MP balance still better in DC or even DoW1?
As for campaign - Soulstorm worse of them all. Dark Crusade much better. And i never saw opposite opinion. Although SS have interesting mechanics and missions.

LAN game require pregious titles too, unfortunatly.

By the way. You can mod few single player related files to fiddle with some campaign mechanics. You get promotion points to hero early. You can change which one region have specific bonus. Both SS and DC. Im personally change Imperial Guard and Sisters statting bonuses (pre build bases). Same pre bhild base available after one region in Dark Crusade - i double it to another standard region.

This region specific are stored in one of the map files. Each map have few files for them. Single player maps have additional scripts files.

Heroes promotions stored in one single file for all factions.

Tgere is also possivility to change campaign army (called Schemes refered to main tabletop games). Someone able to make custom colored armies although im not get how to do it. There is instructions on Steam guides and forums.