do I need to do something or the certificate is valid for any subdomain that is links with my domain ?? I don't want to make any mistake and break what is already works.
Also anyone know how I can add the "www" ? Because for now if I try to access to my website with "https://www.mydomain.fr/portofolio" I have a warning.
The official Certbot - Ubuntufocal Apache guide you linked yourself above has a "Wildcard" tab, next to the tab "default" which shows the non-wildcard instructions. I recommend looking at that first before using $random_guides from the internet: while probably well-meant, the quality of $random_guide_from_the_internet varies from pretty good to straight bad.
Yes thank you you're right I haven't seen the tab.
So I made the wildcard certificate + one for "domain.fr" because "*.domain.fr" doesn't cover "domain.fr" (I say it in case someone is in the same situation than me because as a beginner it is not obvious).