@JeremyCherfas I hadn’t heard of Zettelkasten before. I’ll have a search. Thanks!
ETA: Traced down to its creator’s mostly unstructured intro [luhmann.surge.sh] and a distillation into needed tool support [zettelkasten.de]. But I think it’s a blog comment by a mysterious Daniel [organizingcreativity.com] that sheds the most light on the system in the least words.