I wrote a plugin to easily create collapsible blocks in notes. The blocks are unobtrusively styled, remember whether they were left opened or closed, and can be nested as much as desired. Works on desktop and mobile. Markdown within the title and body of blocks is typically respected.
The basic format is something like this:
:{Title
Body
}:
becoming something like this:
<details><summary>Title</summary>Body</details>
Although it's rather forgiving with how things are formatted.
There are settings to change the open/close tokens from :{ and }: to anything else, plus options to enable/disable the color coding that can be applied to deeply nested blocks in the editor and webview.
GitHub link: GitHub - ntczkjfg/joplin-plugin-collapsible-block (includes screenshots)