If your end goal is to have a better understanding of how blocks work, I would encourage to install the gutenberg plugin instead.
The block code within the WordPress installation itself is not intended to be read by end users nor directly modified. It’s minified to save space.
To answer your question, the code is stored in a couple places:
wp-includes/js/dist/block-library.jswp-includes/blocks/
If you wish to modify a core block, like a block style variation where you can modify the CSS of a particular block, do not modify those files, it’s recommended to place your code in a custom plugin that you would make.
(and depending how much you want to modify it, you may want to just create your own block).