I will need to perform some tests in sandbox mode and therefore, according to the official testing process documentation, to create a developer account.
As I wish this new developer account remains exclusively for testing purpose in the sandbox "virtual environment", I would want to know if the opening process of a developer account automatically creates a live account (using the same email address) or if these two types of accounts can be clearly differentiated?
Indeed, the procedure seems a little bit unclear IMHO because, while trying to sign to the sandbox site seems easy using directlty <sandbox.paypal.com/signin>, trying to do the same for the developer site redirects to <paypal.com/signin> (and not to <developer.paypal.com/signin>, which doesn't exist).
The account that you created to log in into developer is a live account. You can use it to pay real transactions if you want, or you can keep it just for development proposes. Once you log in into the developer portal, you can create sandbox accounts as much as you want. Those sandbox accounts are linked to the developer live account, but with different fake email addresses. As you mentioned, you can use it to log in into https://sandbox.paypal.com/signin and perform all the tests needed.