Some more Info copied from a talk I had with Yiska in Discord:

"Okay so they do basically have something that can be considered a hive queen/king, or atleast the absolute great big one that ensures that they will not go extinct no matter how hard a race/species/world/etc. might try to fend them off and killing their queen will just cause a surviving Usubi to grow and mutate into one and since they are good in hiding in their dens the chance of fully getting rid of them is basically zero anyway.

Rare spawns like Heritors come from them too if Weres unluck out and don't spawn any, so even if a subspecies is considered 'extinct' it'll just end up popping up later anyway. The Leader doesn't need any host species for spawning more Usubis but it's not as fast and not coming with so many Usubis being spawned without one. So it's not too unheard of that someone who wandered off-tracks or got near den openings that they get swarmed by various Usubis and dragged down into the largest cave openings and thrown infront of the Queen. Or sometimes a Were hears the call of the Queen and can decided wether or wether not they just take a roomie, comrade, spaceship engie, etc. and bring it along, they have more freedom leeway, since they're basically free agents.

Anyway, once someone is dragged infront of the Queen they first get checked out for potential of what sort of Usubi they might be best suited for spawning, how much they can spawn and if it might not be a good idea afterwards to order an Heritor ( regardless of age ) to turn them into a Were as future investment. After the assessment the Queen swallows the person, or multiple, to invade their body with the approptiate slime without moving too much or anything, they don't like moving too much due to size.

Once the person has been properly inflated-
they are placed back outside the Queens main body and watched over by Usubis while the Queens slime inside them forms into proper Usubis. That can be a very rinse and repeat process, so even the freshly spawned new Usubis left the hosts body they might get a bit of rest ( Usubis come with free nourishment ) and then get dragged back to the Queen.

If they don't get turned into a Were by the Queens order, they get their memory wiped by her and brought back to the surface. If they ended up liking a host out of sometimes random reasons, maybe they like their eyes, they might just end up becoming permament den guests.
That's so far all to the main Hiveleaders."