CloHo Theory Time! Under the cut because Long
Comfrey can get from Zood to Zern and back
You travel from Gath to Zood through the Effulgent Biangle
There is no known way to get back to Gath
The Device that we found in the South Pole Station reacted to the Biangle
Device is described as a ceramic tile hovering in a crystal case-like thing
When we found it, the was oscillating (moving back and forth) with the motion of the water
as such I call it a CCO for Ceramic Crystal Oscillator
The sides have "light" and presumably "dark" written on them in the Ramansu symbols
the light side corresponds to Effulgent energy (Zood), and the dark corresponds to Calefactory energy (Zern, Straka)
At the Effulgent Biangle, it lit up towards the light side and let the crew go through to Zood
There were similar stone rooms in the South Pole Station and Ramansu Power Station
they were both made of Gathie stone, had large pits in the middle, and seemed to be unnatural to those locations.
We could hear a CCO at the bottom of the well at Ramansu, but I don't think we got it
Things we don't know, but are probable:
Comfrey can get back and forth from Zood/Zern to Gath [Evidence: her showing up when Marya confronted Straka]
I think that Comfrey's using the CCOs to get to the different worlds by harnessing the different energies of the Biangles. In both places that we've seen her do actual work (Ramansu and South Pole), we find these CCOs oscillating at the bottom of these pits.
There's a thing with oscillators called a resonant frequency. Basically, when you drive an oscillator at this frequency, the amplitude gets much bigger. The particular frequencies depends on the system, but there definitely can be more than one.
I think that the CCOs have resonant frequencies that are the biangle energies (which ones probably depends on the construction), and when resonated, activate a portal to the other world. The rooms/pits that we found at both Ramansu and the South Pole probably aid in this resonance and/or contain the resulting portal.
If this is true, it provides an explanation for why Comfrey was at Ramansu in particular- it has a steady supply of Calefactory energy ready to resonate and get her to Zern.
This doesn't explain how she would be getting back to Gath, however. The crew hasn't seen an energy that is particulary Gathie, but that doesn't mean there isn't one. Comfrey might also have figured out how to channel the fact that she's from Gath, or it might be a completly different mechanism entirely.
Also I'm pretty sure we're playing a cosmic game of rock paper scissors where Calefactory beats Effulgent, Effulgent beats Obtenebrant, and Obtenebrant beats Calefactory.