It's a bit complicated for me cause it's an overall very positive experience, but my memory falters a lot, especially when I'm small, so I don't remember many specifics about regressing.
I more often than not regress with my cg and fiance, since it's comforting to have someone looking over me while I can just relax without any big stressors on my mind.
It's hard to tell when you're 'in it', since for me, I just slip right into it sometimes. I start getting more comfortable, more giggly, sometimes more sleepy and cozy. I become unfiltered and I just make lots of sounds. I stim a lot more too, especially if I'm in VRC with my cg and fiance.
I used to regress alone, and when I did, I'd more often than not indulge in more nostalgic experiences. I have a lot of computer games on my desktop that I used to play when I was a kid, so that is my first go-to when I'm alone or not doing another activity with someone. Here's a link to all the games on my desktop, most of which are these games.
Sometimes it can also be a bit upsetting to regress, since it's harder for me to express wants or needs when I feel vulnerable. I tend to just go with whatever everyone else is doing, but these nervous moments for me are usually short lived. It's a very different experience for everybody, nobody experiences it the same.