How to create block to list all children of an entity?
Finally, I figured it out!
First, I learned more about contextual filters and relationships, and what they each mean and do (relationships basically relates to the FROM query, contextual filters to the WHERE query)
https://www.youtube.com/watch?v=mRWtpcUm9pI
Another user had a similar question and solved it like so:
https://www.drupal.org/project/views/issues/1724396
So all I had to do was:
/admin/structure/views/add

show content of type &quot;Locations&quot;
Create a block

Then under &quot;advanced&quot; add a contextual filter, check &quot;parent_location (field_parent_location)&quot;, press the button to &quot;Add and configure&quot;.
Then under &quot;When no filter is available&quot; say &quot;provide a default value&quot; and select &quot;content ID from URL&quot;
Hit apply, save the view.
Under /admin/structure/block place the block to the content region and make it visible for Locations.
This was super simple! :-)