Editing the collapsible sections isn't handy and neat. Especially, if you used some other note-taking apps, you clearly see what I mean. If there is an easier way to create collapsible sections, please inform me.
You should be able to toggle collapsible sections - if you can't, you're likely in the Rich Text Editor instead of the Markdown Editor. Make sure you're on the left/red side of the toggle in the image below, which swaps between the two - then use the blue button to swap between the editor and the webview, which is where you'll be able to view and toggle the collapsible sections.
You may also be interested in my collapsible block plugin, which allows for the creation of these blocks without having to type out the HTML, and with a few other features. So instead of:
<details><summary>Title</summary>Body</details>
you'd write
:{Title
Body
}:
Note that my plugin won't work in the Rich Text Editor either - most won't. I just stick with the Markdown editor 100% of the time, and I think that's pretty common.
I know it man, I just wish it was more polished. I don't want to toggle anything to add a simple collapsible section. It should be as easy as making a text bold etc.
Developers should take this as a user advice because nowadays adding collapsible section to the notes is in high demand among note-taking apps.
There is also a code folding plugin for headers and lists "like in Obsidian" @bwat47 mentioned, It is:
or with a little bit more options:
Maybe this can help you. It will always be helpful to take an deeper look into the Joplin Plugin Section, there are may helpful plugins. Also there are partly option for custom css formatting.