You can add your own custom domains to your project under Settings → Domains. There is always one primary domain. All other domains will redirect to the primary domain with a 301 redirect.
For each domain you add, the records will be checked to see what needs to happen to make this domain work. You will get detailed instructions on which DNS records need to be created.
Note that you can add domains to your project at any given time. However, these will only have effect when your project is published to production. See the Publishing section for more details.
When you add a new domain to your project, you can check Create with 'www' prefix as well, which is on by default. This lets you create the domain with and without 'www' with one click.
Note that this will result in 2 separate domains. Technically, they are 2 different domains and each needs separate threatment to point to the project. This also allows you to choose whether you want the domain with or without 'www' as the primary domain.
For non-primary domains, you can optionally enter a path. When left empty, the domain will just redirect to the primary domain. But when you enter a path, that path will be appended to the primary domain when redirecting.
Let's say your primary domain is mycompany.com, but you also have the domain mycompany.fr and you have a French language on your site. Then you can decide to enter the path /fr for the domain, so mycompany.fr is redirected to mycompany.com/fr.