You can find informations about non-human alters e.g. here: https://did-research.org/did/alters/non-human
Communication between alters/parts is ensured by creating new neurone pathways in the brain through mind exercises (like anything you learn, really). If the stroke damaged some pathways, you need to create new ones around the damaged areas, just like you need speech therapy to re-learn to articulate properly, and so on.
And you are all doing a nice job at it!

Oh also, @Kit and pals: fusion cannot happen if you do not want it. Yuna and Juno are curious about what it is, how it works etc. but it does not mean they absolutely want to force it upon everybody inside! It's completely okay if you (Kit etc.) want to stop at "functional multiplicity". It's legitimate and it's a perfect good way of healing from traumas and dissociation.
I understand that you are scared and everything. You can breathe. Nothing can be forced upon you regarding fusion. Fusion cannot happen spontaneousely if you do not want it.