You set up specific styles for the paragraph following each heading style. For an example of this, take a look at the styles in this template.
Outline Headings Global Stylesheet for Word

This has Body Text 1, Body Text 2, etc. as styles for following paragraphs. Might be BT1, BT2, etc. It has been a while since I looked at it.

Each heading level has an associated body text level. The only difference in the body text styles is the left indent, which is set to match the corresponding heading style.

By the way, in setting up your headings, make sure you follow the instructions at How to create numbered headings or outline numbering in Ribbon Versions of Word by Shauna Kelly
This is very important.