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
I found out about this a couple years ago when my wife started a conversation with me like “do you know some people can’t picture things?”. I had several follow up questions because I thought it was just a figure of speech for the first ~30 years of my life.
My internal voice is exactly like me speaking out loud. If I don’t “speak” in my mind there’s nothing, just like if I don’t speak I’m not saying anything out loud.