A lot of Miko's lore is explained in her debut video which is one of the only videos still remaining on her channel from her pre-Hololive days. Watch the video.
https://youtu.be/lMPhHXJbPEQAlso, Miko no Tsutome is no longer on her channel but I did find this english subbed version a while ago while looking for any info about the short series. A lot of stuff is explained in this show.
https://youtube.com/playlist?list=PLl7bGxjs0EOkqa_Chq7-tk8lC3xkdi8L2 