Stop using heading styles for your body text!
The collapsible headings are those that have an outline level other than body text.
This is built into the program and you cannot turn it off. You can set an option to un-collapse them all upon opening a document. This is a setting on your computer, though, not on the document.
Jargon tip:
What is the difference between headings and headers?
By default, Headings 1-3 are included in a Table of Contents. You can change that.