Netlify allows you to either purchase and configure a domain name through them or bring a domain name from another registrar. If the domain name is from another registrar, you can allow Netlify to manage the DNS settings, but you don't have to.
Once you've added a custom domain to the site, you'll want to make sure that there's an SSL certificate provisioned so that your site is served over HTTPS. Netlify offers free SSL certificates through Let's Encrypt. They will also automatically renew them for you so you don't have to manage anything at all.
If you want to add a new domain name to your app, go ahead and do it now. If you just want to see how to do it, follow along!