Obsidian is a personal knowledge management tool based around a Markdown editor. It has a ton of features and many plugins to customize the experience of working with it. If you decide to try it out, don’t be discouraged and feel like you need to dive into the plugin ecosystem. What comes with Obsidian out of the box is great.
It’s free to use and available for all major platforms. There’s a paid Sync service (which I use) that works great and does end-to-end encryption of your data. You can also sync using iCloud, git, and others. There’s also a paid Publish service that lets you put your notes on the web, if you wish. (I’m personally using Quartz to put this site together.)