Note: this list is generated by looking for the string for this theme hook, so it may include some references that are not actually using this theme hook.

LayoutRendererFlexible::renderEditor in core/modules/layout/plugins/renderers/layout_renderer_flexible.inc
Perform display-level render operations.
LayoutRendererStandard::renderLayout in core/modules/layout/plugins/renderers/layout_renderer_standard.inc
Perform display/layout-level render operations.
layout_get_layout_template_info in core/modules/layout/layout.module
Load the information of either a single layout template or all available layout templates.
layout_list_page in core/modules/layout/layout.admin.inc
Output a list of pages that are managed.
layout_preprocess_layout in core/modules/layout/layout.theme.inc
Prepares variables for layout templates.
layout_reset_caches in core/modules/layout/layout.module
Reset all caches provided by Layout module.
layout_theme in core/modules/layout/layout.module
Implements hook_theme()