They're part of the building blocks in the back of the first Alien Archive. You combine various templates/types with an array (combatant, expert, or spellcaster) to create a monster.
So a dragon template can only be applied to a dragon type (which may also dictate a certain array). Similarly, various undead specific templates dictate they be applied to the undead type. Many of these specific creature templates also are limited to one or two arrays.
You wouldn't apply a "true dragon" template to a dragonkin or drake, because those are effectively their own templates (just not with generalized rules for building other varieties) already applied to the dragon type and a relevant array.
It's up to the GM whether he wants a certain undead creature to apply to a certain underlying original creature type unless the template provides otherwise or you want to just ignore that to make something cool or unique. Certainly the default for these undead is a humanoid, but there's no reason you can't have an undead monstrous humanoid or other type of PC race. Or even a dragon, with some mix of undead and dragon template powers you choose to give it.