Here is video: How to add the Custom Layout Update on Magento 2.3.4?
In Magento 2.3.4, there is no more the Layout Update XML field in the admin panel. Instead of it, you will find the Custom Layout Update dropdown on the CMS Page Edit, Category Edit, and Product Edit pages. It was done to increase the security level of the system.
Now, to make a custom layout, you need to create a file and place it on the server in the module or theme folder, for example, app/design/frontend/Vendor/Theme/Magento_Catalog/layout. Then you will be able to choose its name from the list in the admin panel.