How do I insert text below a headline?

Short answer: use a comment as placeholder in your note, below the headline in question, and then employ the Notes / Insert Text At Placeholder In Note or Daily Notes /Placeholder In Daily Note action.

However, if you don't want to or can't use placeholders, you could opt for a Notes / Search And Replace In A Note action instead, and make use of its regular expression option.

Example: Assuming your headline is "## My headline" and you want to add "something something" right below it:

• Search term: `/(## My headline)/`
• Replacement: `$1\nsomething something`

The replacement contains the captured headline, a line break, and the string "something something", i.e. the headline is preserved.

For more info about these actions, see Notes / Search And Replace In A Note or Daily Notes / Search And Replace In Daily Note.