Hi Thanos,
I believe the article linked below can provide an answer. Although it doesn’t mention audio effects specifically, I believe they are handled the same way. I’m curious if this is the case, can anyone else provide clarity?
“By default, when you create or customize any Final Cut Pro effects, transitions, titles, or generators in Motion, that content is stored in the Motion Templates folder located in your Movies folder. You can also store Motion content inside a library—for example, to move a project or library to another Mac, or to back up and archive your work. You may also want to store Motion content in the library if you’re working on a project with others and using shared storage.”
Final Cut Pro X: Manage Motion content storage locations
https://support.apple.com/kb/PH26157
Instagram.com/FinalCutProHelp