Category widget/block

Hi there,
I’ve just started looking at Gutenberg and am quite impressed.
I decided to look at the Category widget as I’ve just finished my own in php.
How and where do I edit the block code so I can show the posts for each category?
I’ve added the block to the reusable blocks and the Code Editor has changed from:

<!-- wp:paragraph -->
<p>This is a list of post categories:</p>
<!-- /wp:paragraph -->

<!-- wp:categories {"showHierarchy":true} /-->

to:

<!-- wp:paragraph -->
<p>This is a list of post categories:</p>
<!-- /wp:paragraph -->

<!-- wp:block {"ref":1244} /-->

Many thanks
PS I’m a bear of little brain (and a stinking cold) so answers in idiot’s language please