In the site we have the next Custom Block types:
For almost cases you have to use Basic block type.
To add a new Basic block go to Structure → Block layout → Custom block library, and after click "Add custom block" button and select "Basic block"
There are two fields "Block description" and "Body"
In the "Body" filed feel free add any content like text, images and etc.
After you created a block you need go to "Block layout" page (Structure → Block layout) and choose a region for this block and click "Place block" button
After it you have to see a list with all block in the system
Find needed block and click "Place block" button
In this place you can add settings for this block like Title, Visibility and etc.
Also, you can reuse one block for many times
An important note:
Be careful with "Block layout" page. This page has settings for each block positions for the system, also we don't keep block settings in configs files (it means you can't revert changes)