Obsidian
I love to use Obsidian. I've used Obsidian for years on multiple computers and mobile phones. It's free to use.
Obsidian is not only my favourite digital knowledge system but what I use to make and publish this digital garden.
Obsidian is software that can be very simple or very complex: it's what you make of it.
To begin with, Obsidian is a place where you store information. The data is, to begin with, saved on the device on which you use Obsidian. In other words, nobody has access to your data than yourself.
Obsidian make money from offering two things: sync and publish. Sync lets you encrypt and synchronise your Obsidian data to Obsidian servers; not only does this mean that your data is safe from accidental deletion if your device is ruined but it means you can use several different devices and make sure that your data is always up-to-date everywhere. Publish lets you publish an Obsidian vault to the internet. Obsidian Help is an example of an Obsidian vault that's published to the internet.
So, what else?
Anybody with programming skills can create their own Obsidian plugin. A plugin is something that's used to enhance Obsidian in some way.
I maintain a list of my favourite Obsidian plugins where I describe their benefits.
The people who develop Obsidian are, as far as I know, nice people. They allow plugins that 'go against' their business model; for example, I don't use Obsidian's paid-for way to publish and maintain this digital garden. Instead, I use Ole Eskild Steensen's brilliant Obsidian Digital Garden plugin that lets me keep the garden for free.