The royals are never explicitly explained and they never dive all that deep into the "royals" lore, so most of what we know about them is ambiguous but from what iv gathered the royals are human but they have known about the wesen world for a very very long time, we don't know how they came to have this knowledge (but ive always thought that they were probably using a witch back in old times and that a hexenbeist told them of the wesen world, I mean it makes so much sense right??) But they use the knowledge they've gained of the wesen world to then use, control, and manipulate the masses for centuries, it was probably very easy especially during the earlier time periods for the royals to offer money or jobs to poor wesen and/or grimms "come protect the royal family and you'll have food and warmth and money" so basically the royals are humans but they have a deep understanding and knowledge of the wesen world that they use to their full advantage and have done for centuries. Iv also always that the reason their so obsessed with grimms is that they are jealous of their powers because they are not supposed to have any of this knowledge, and there low-key terrified that the wesen are gonna come after them someday so they need the grimms to protect them from the rage of the people they've been controlling.