Adopting is putting an old original character that someone doesn't want anymore up for sale and at some point, someone else will be up to paying currency in order to claim ownership to that character.
Sometimes, there can be people with original characters that have character development, but don't appear in any stories. This could mean that the character was recently just created, or that the creator is not a writer of fanfiction.
Toyhouse is a website where users can post about original characters to explain their appearance, ages, backstory, etc.
I can't speak for anyone using Toyhouse as I've never used it and kind of learned about it not that long ago. With that said, I know a lot of friends and people who create characters do so because it's fun to draw and be creative and put that on paper. But fleshing out a full character or even a story takes a lot of time and patience. Sometimes people don't want to dedicate so much time on just one character. They have all these other ideas and they want to put them on paper to when the inspiration is still there. And that's fine. No one needs to be creating a final polished feature-length book for all of their OC.
I'm still writing this one story that's been on-going for years. But sometimes I just want a break to do so other writings or drawings and not be confined to just that one story. That would be boring to lock myself away from other ideas I have because I'm not finished my previous one.