This functionality was removed in Magento 2.3.4 due to a vulnerability which allowed admin users to execute code via the custom layout field. There is more info about it in this blog post.
The solution is now to manage this with XML files in the codebase, the file name must be in this format - catalog_category_view_selectable_<Category ID>_<Layout ...
There is no way to apply an a layout update to multiple layout handles with just one XML file.
What you could do, is add a custom layout handle to all CMS pages where you want the layout to be updated. Then you can create one layout XML for that handle which is applied to all pages with that custom handle.
To do that you need to write a plugin for the ...
If you want to add related product and cross sale product in tab then use below code.
First, create a file with name: catalog_product_view.xml and save it under: app/design/frontend///Magento_Catalog/layout/catalog_product_view.xml. Add the following code into the file
Are you just looking for the file which holds the HTML for each item rendered in that foreach list?
You can find that here:
You can create a version of this in your theme for your custom HTML here: