I just copy and paste, or transclude. I prefer that, since chances are not all the notes are actually relevant to what I want to write about. Personally, I find the process of curating, reordering, rewriting content/notes/ideas is an important part of the thinking process for me. YMMV 
I do wish there were a way to throw a bunch of notes on a workspace, or within a single note, and move them around the way you can with physical index cards. That would be a big game changer for me when writing in Obsidian.