Kiara explained that when she meets new members, she introduces herself as Kiara. Kronii as well once accidentally said said Fauna in public instead of referring to her with another name (iirc), so going off that, I assume that they just talk to each other useing the Hololive names.
I could see some exceptions tho, mainly Kiara and Calli, who used their Roommate handels for so long by now, it might feel a lot more natural than their real name or the Hololive Name. But thats just a hunch.