FCPX old plug-ins don't work

That specific CoreMelt product is a plugin, not a template. Templates reside in those folders. A "plugin" technically is coded and installed at the OS level. It won't show up there. Contact Coremelt about this, they're friends and have good tech support. You probably just need to update your templates and plugins.

can't find them in the Motion folder (Effects, Titles, Generators etc)

Motion folder? Motion Templates folder? Where are you looking? Please show me a screenshot.

C2 LUTx appears to be a Coremelt product. You could try asking their support about it. https://support.coremelt.com/hc/en-gb/requests/new

Please show me the inside of Effects.

You’ve probably deleted the installer package. Most developers who use system installers bundle it with an uninstaller utility. You could try downloading it from CoreMelt.

CoreMelt has a product management app, like many plugin developers are starting to use. Something Logic Pro users have been doing for years.

https://coremelt.com/pages/downloads

Usually an uninstaller will remove other dross like plist files and other caches that are on the user library.

This was placed there by the installer and it looks

It looks like you have a clutter of duplicate and unused plugins. Since this is a good opportunity, why not narrow it down to only the necessary plug-ins that you use on a regular basis?

Store plugins that you do not want to delete in a separate folder. Delete everything else →Please open the “Unresolved Folder” in one year and reconsider.

I think the first priority is to reduce the amount. If you don't reduce it, plugins will continue to go missing. You must know what files are in which hierarchy. Please let me know if you want to reduce it. Let's organize it together.

Why is the same folder in Effects? You are not organizing your hierarchy in an orderly manner.

What is included in the second Effects? Please show me until the end.