Astroid by default provides 4 different types of layouts, that are available in the layout manager.
Each of these layout’s core files are located in the following folder.
JOOMLA_SITE_PATH/libraries/astroid/framework/elements
If you’d like to modify the structure of one of these elements, you can do it by copying the folder to the template folder.
Let’s imagine you want to modify the banner.php layout, which is located at: JOOMLA_ROOT/libraries/astroid/framework/elements/banner/banner.php
You can copy the whole layout folder to the following path: JOOMLA_SITE_PATH/templates/YOUR_ASTROID_TEMPLATE/astroid/elements/banner/ and modify the layout file banner.php to fit your needs.
See Fronted Overrides in Astroid for more.