hii!! I can help you with this query 🫡 the first time it was mentioned was in the "find trouble inside" video that they released before troubleshooting came out talking about each of the songs, and when they got to talking about walking to the moon jiseok was saying that they were having trouble with the melody for the chorus, so he told jooyeon to grab the mic and just keep trying, and again, and again, until they landed on something. It's here at about 13:32 (in case the time stamp on the link doesn't work): https://youtu.be/mDU2pfvr93U?si=WRlNr3KY0k-HdJcx&t=812

there are a couple other times they've talked about it, referring to it has jiseok having "squeezed" the melody out of jooyeon... I think they talked about it on a radio show or livestream maybe?? but while my memory is fairly good I'm like godawful at actually keeping track of clips and stuff, but one mention I can find of it is here: https://x.com/jooyeonsfan/status/1791286695310299178 and interestingly both of them have referred to it as their favourite song at one point or another, especially jooyeon... jooyeon reeeaally loves that song...

so, basically, walking to the moon is jooseok's biological daughter