How to customize Strapi backend (content-manager) controllers, services and validation?

hi, I’m trying too, to customize a create and update method used in the backend of strapi.
I saw that I can customize the content manager plugin like :
extensions/content/manager/controllers/collection-types.js → and customize the async create(ctx) method for example.

But I just want to customize one specific content type, not all the contents types.
I could use a if(myContentType) to do that inside the create method, but I do not think it is clean.

How can I do that ?
Thanks