You can place the code in your theme’s functions.php file or in a custom plugin. This link explains the differences. I like to use custom plugins, that way, if switching themes, my customizations remain. Using the Pluginception plugin (not necessarily supported by ACF, just a tool I like to suggest to simplify things) allows you to quickly create a plugin for this purpose.
I can’t fully understand your exact use case. Please explain it step by step with full details 😉 This thread might get you going otherwise.
