Unless otherwise specified, the title of the document's root element, such as book, is handled by the template that starts with this line in html/titlepage.xsl:

<xsl:template match="title" mode="titlepage.mode">

This template outputs an H1 element. You could add a customized template to match on book/title in that mode to change just book titles.

A part title is handled by the template named division.title in the stylesheet module html/division.xsl. That template was specified in the titlepage spec file. You could customize that template, or customize the titlepage spec file to use a new named template instead for part title elements.

A chapter or appendix title is handled by a template named component.title in the stylesheet module html/component.xsl. That template is used for other component-level elements such as bibliography that might appear inside a section. So it first computes a level variable based on its ancestry, and then adds 1 to that to use in an HTML heading element. Since chapter cannot appear inside a section, the level value is 1, and so an H2 heading is produced.

A section title is handled by a template named section.heading in the stylesheet module html/sections.xsl. That template is called from a template named section.title with a template parameter named level computed by using the template named section.level.