https://docs.magento.com/m2/ce/user_guide/cms/block-position.html

Positioning Blocks

The code that controls the page layout and placement of blocks is written in XML Widgets make it easy to position a block at a specific place on the page, and even for a specific product or category without writing any code. You can choose each option from a list, rather than trying to remember all of the possible combinations.

The following list shows the locations by page type where blocks are typically placed. To learn more about how areas on the page are defined, see: Standard Page Layouts.