Custom code

I have a selection menu that selects from 6200 listings in the database. Each of these is a WordPress page, not post. There are more pages to the site that are not selected by this menu. I am upgrading and the old theme could not be used in the newest versions of WordPress. It has never been upgraded to work with the new WP. I chose Customizr after trying many others.
Selections from this menu need extra code to show some data from the database on each page that is specific to the individual page. I was able to accomplish this in the previous version by using multiple themes and a theme switcher – much easier than trying to deal with child themes. The old theme allowed me to duplicate and rename it and add the code to the new theme. With Customizr, this does not work.
I tried making a child theme and could not get this to work, either partially due to how complex dealing with a child theme is and/or not being able to find where to place the code so it would appear where needed. There was no readily available succinct information on how to do this with a child theme.
Several plugins were tried and either the code won’t execute or the plugins didn’t work at all. One even puts in the code twice without executing it and even duplicates the graphics, but does not execute the php.
The code must display its results immediately after the title. It contains information related to the particular title and is needed on each page selected by the selection menu.

Where does this code go in order for it to even show up, and How to get it to run as required is the issue. It is to run only on pages selected by the selection menu.