This is a level of detail that the official materials didn’t get into. My assumption is that there are two states, a feudal monarchy, and an administrative monarchy and that each domain is somewhere in the transition between the two. Then there are other types of government that can influence what the government of a domain looks like. Merchant republics, noble republics, despotisms, &c, &c.
A feudal monarchy is one where the ruler relies on their underlings to perform tasks locally. For example, Thuriene relies on the Earl of Freestead to protect the people of Freestead against the goblins of the Five Peaks, to be the first response to a large scale invasion, to maintain fortresses in the area, provide local justice, and so on. Holdings, military units, and other state instruments would be staffed or even run by the local Earl or his Lords and these would all acknowledge the Thane as their rightful overlord. The Thane may own little if anything in the provinces, and is entirely dependent on her vassals in any domain she runs in a feudal style to execute her will. In this case the people generally have more loyalty and attachment to the local lords or the provincial lord, and finally some regard for the domain ruler. But, the holdings tell if the lords are acting for themselves or if they are acting for their liege. In a domain like Talinie, if the law holdings of Freestead say Thuriene Donnals, then even if Torias is the law, he is upholding his vassal obligations and Thurien’s will is done. If the local province rulers have their own holdings, they act for themselves regardless of oaths of vassalage.
In an administrative monarchy, the ruler has a staff of royal officials who carry out her will. In this case, Thuriene would have royal officials in Freestead to carry out her will and staff her holdings. In this case, the Earl of Freestead could effectively be nothing more than a rich private citizen with special privileges. He might earn extra money as an official in the Thane’s government, or he might just manage his own private fortune. In this case, the Thane may own little if anything in the provinces, but have legal rights to act as a sovereign. In this case the liege is the only direct power, but the nobles have plenty of social power and respect.
In either case the domain owner can own plenty of land, but doesn’t have to. Owning land makes you a direct landlord, and you are always on the administrative side of things when you directly own land, and may or many not be feudal when you don’t
The listing of holdings for the NIT and the descriptions in Talinie say that it’s the NIT that really controls the law holdings, not the landed side of things, so 1) if they ever separated, the NIT would probably control the law, not the landed ruler, and 2) the people you see as judges, justices, and sheriffs are not nobles, knights, and their officials, but priests and their officials.
A domain like Talinie can also have aspects of being a theocracy, where the ruler can be forced to do the will of the temple doctrine rather than imposing their will on their own organization. A figure like Torias Griene can wield a lot of power this way, because if he can convincingly claim to know the word of Haelyn, he gets his way. In a realm like Boeruine, someone can argue that Haelyn wants this or that, but the way the Archduchy works, the Archduke’s will is done, not Haelyn’s. A temple realm could be either way, though some will accuse a ruler of being corrupt if they impose their will against the clear doctrine of the faith. Medieval Popes had this problem all the time, and a some religious rulers will just say that they are the boss and they know best, like it or lump it. Regency points spend the same no matter how they are used. Depending on what is exactly going on such a ruler might or might not have problems behaving this way (too many issues to go into on a post on nobility and the law).
A state like Endier might look like a hybrid of administrative monarchy and merchant republic. Ultimatly there are no republics because it’s a game of birthright, not government by the people, but there can certainly be republican aspects to a monarchy. A domain where the ruler has clear powers, but must get the support of a parliament, estates, or guild counsel to undertake certain acts. This could be a simple as requiring a free diplomacy action to be successful before getting high taxation. A lot of realms start out in a state of crisis, as described by the PS’s and getting the consent of noble counsels, assemblies of the commons, and such can be necessary until the ruler has established themselves as the guy who solved all these problems.
So, based on what the DM’s tastes are – knightly, medieval, feudal, or divine right, absolutist, one man rule, or divided government, parliaments, back room politicking - and what kind of realm is described, and what the DM knows about how governments work, you can devise different kinds of governments.
As a general rule, I imagine that most states are somewhere between feudal and aristocratic monarchies, and admix other kinds of governments as needed to create the right feel.