You can absolutely open and edit a .docx file created in Word 2019/365 in Word 2010. There are some features that will not function in the earlier version, but it does not sound like you are using anything like that.

If you are writing a book in Word, it is very important that you learn to use Styles and that you base your chapters all on the same template using the same styles if they are in different files.
Importance of Styles in Word


I strongly suggest spending a few minutes at least reviewing Basic Concepts of Microsoft Word - from Shauna Kelly. It will save you days or weeks of frustration.