Structure -> Block layout -> Custom block library
The "Custom block library" tab is located next to the "Block layout" tab in the block layout. Here you can create your own custom blocks and edit the contents of the previously created custom blocks. For example, a custom block can contain images, free text or links to content. Here is an example of a custom block: The block has the title "Key data" and displays two hyperlinks to a basic page with the title "Objects" and a second hyperlink with the title "Measurements". Both Basic pages are generated from views:
Next to the link "Blocks" there is a link "Block types". A list of all block types available for the Custom block library appears here. In addition new "custom block types" can be added ("Add custom block type") and new fields can be added to existing custom block types ("Manage fields").
The newly created block can then be inserted and configured in a region under Custom block library.