Each folder I have is organised by topics.
Some notes/subfolders are relevant across multiple folders, so there should be a way to create a shortcut/symlink/junction. However, this should be natively done within Obsidian, with a “create shortcut”.
Creating a symlink by OS is not recommended (Symbolic links and junctions - Obsidian Help) and is complicated when the vault is shared/synced between OS’, so the solution has to come from Obsidian itself or a plugin that reads some properties of a regular .md file (but this seems substandard).