For me there’s two separate participants, a ‘talker’ and a ‘listener’. My mind identifies more with the talker, because that’s the one that has agency. Since there are two participants, both of which are me, I talk in 1st person plural (‘we’ve got to do …’, 'we thought about this earlier’). I stopped being afraid of being alone after I started having an internal dialogue around the age of 11, since having a second participant in the conversation meant I was always in company.
Edit: Wow, looks like there’s a lot more diversity in this than I was expecting
For me the internal monologue is exactly the same as my ‘external’ monologue when I tell people about myself or do something together with someone and explain my actions. So it’s always first person singular, for example: “I’ve reflected on this multiple times but still don’t quite understand it”, “Okay, I need to turn right now” or “God I’m so freaking tired of this shit! I’m done. Fuck them all”. There’s no internal split and if I was saying what I’m thinking out loud in front of someone, it’d sound completely normal.