@huellyouhappy There is no direct way to merge 2 studies within Lichess.
There is no direct way to move or copy one chapter from one study to another.
If you want to do this then you have to export and import PGN files.
1. Some information like the "Pinned chapter comments" will get lost during PGN export
2. When you import a big PGN file then only 20 chapters will be imported. If there are more chapters in the PGN file then you must manually edit the PGN file before you can import the chapters auf chapter 20.