Paragraph formatting (hopefully applied through styles) is the key.

• Keep with next
• Page break before
• Keep lines together

The effects of these can combine. For instance, most heading paragraphs are formatted both to keep lines together and to keep with next. Many executive summary paragraphs are formatted to keep lines together.

Sometimes a style with one or more of these attributes is applied to a paragraph (say a heading style) and then someone decides it isn't a heading and uses direct (not style-based) formatting to get it to not look like a heading. Then text from that paragraph gets pasted elsewhere in the document carrying that hybrid formatting with it.

It looks like you have a big project. I would advise learning more about Word formatting and styles before you go too far into it. Otherwise you'll be doing things you need to undo later on. Start with Styles and paragraph-level formatting.
Understanding Styles in Microsoft Word