This is for those interested in the original story from which this drama was adapted and for ARMYs familiar with BU universe who want to refresh their memories. I myself forgot a lot of details, especially from HYYH notes.
1. Original BTS MVs and short films that are part of BU/HYYH storyline (YT playlist)
2. HYYH Notes (this is from album booklets and published books - The Notes 1 & 2)
3. Save Me webtoon
You can find more additional information on this site: https://www.usbtsarmy.com/bangtan-universe-guide. It's super organized and easy to navigate.