No, actually, I should be in the right one.
There are two different websites sharing the same name, but only one domain exist, the one on Shopify is all good and as no need for any type of modification.
To access the PrestaShop back-office one, it requires the domain name that have been given to the other site to exist.
What brings me to the point of my question, How to access a PrestaShop back-office when the domain name has been changed, or given to another one if you prefer.
Like : https://myShop.com/adminxxxx is not accessible because my clients asked me to make "MyShop" on another site, and then asked me to re-enable the original presta back-office while keeping the new one online.