Roam
Discord
I think of all of these tools as "personal hypertext notebooks" - basically taking what is possible in wikis (organizing by means of linking, hypertext) and lowering the barrier to entry (no need to spin up a server, can just download an app and go).
The common thread across these notebooks then is allowing for organizing and exploring by means of bidirectional hyperlinks between "notes": - In Obsidian each linkable note is a markdown file and can be as short or long as you like - in Logseq/Roam and other outliner-style notebooks, you can link "pages", and also individual bullets in the outlines on each page.
In this way, the core functionality of these tools is similar to a wiki, but they do leave out a lot of the collaborative functionality that makes wikis work well (granular versioning and edit histories, talk pages, etc.). So for folks like <@305044217393053697> who are comfortable with wikis already, they add marginal value IMO.
Their technical predecessors in the "personal (vs. collaborative) wiki" space include TiddlyWiki and emacs org-mode (and inherit their technical extensibility: many users create their own extensions of the notebooks' functionality. an example is the Roam Discourse Graph extension that <@824740026575355906> is using).
These tools also tend to trace their idea lineage back to vannevar bush's Memex and ted nelson's Xanadu.
These tools are still not entirely mainstream compared to tools like Notion, which is related to your experience trying to learn more about the tools - so they tend to have a steep learning curve!
IMO the best way to get a feel for what they are is to see some examples/videos.
I like this video for an overview of Logseq: https://www.youtube.com/watch?v=ZtRozP8hfEY&t=6s
I describe Roam and the Roam Discourse Graph extension in this portion of a talk I recently gave: https://youtu.be/jH-QF7rVSeo?t=1417