The Zettelkasten has recently become popular in the digital sphere as a form of "non-linear note-taking" with tools like Roam Research. Many other digital implementations have sprung up, like Obsidian, Logseq, and the tool I use, 🍂org-roam.

A Zettelkasten (German: "slip box", plural Zettelkästen) or card file consists of small items of information stored on paper slips or cards that may be linked to each other through subject headings or other metadata such as numbers and tags. It has often been used as a system of note-taking and personal knowledge management for research, study, and writing.

–Wikipedia

One researcher famous for his extensive use of the method was the German sociologist Niklas Luhmann (1927–1998). Starting in 1952–1953, Luhmann built up a Zettelkasten of some 90,000 index cards for his research, and credited it for enabling his extraordinarily prolific writing (including over 70 books and 400 scholarly articles). He linked the cards together by assigning each a unique index number based on a branching hierarchy. These index cards were digitized and made available online in 2019. Luhmann described the Zettelkasten as part of his research into systems theory in the essay "Kommunikation mit Zettelkästen".

–Wikipedia