There is no single correct way to achieve this. The work to do this lies outside of WordPress itself. The general process requires the following steps.
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Point the new domains/subdomains to your WordPress site. I’d recommend using A records as CNAMES have gotchas for new players. Never use a CNAME on example.com (but you can usually use it OK on www.example.com)
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Configure the webserver to recognise these new domain names as being associated with the virtualhost of the main site.
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Expand the SSL certificate to include the additional names (ie add Subject Alt Names to the certificate) and get the browser to pick up on this.
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Ensure that WordPress uses its preferred canonical name – this is the default setup and will ensure redirects happen seemlessly.
Step 1 typically happens with your registrar (to be more specific, it happens on your nameservers and its common for the registrar to provide you with nameservers. There are exceptions.
Step 2 happens on the web server. Most control panels will have a way of adding these.
Step 3 happens on the web server. If you are using a control panel they will either expand the existing certificate or create a new one with extended attributes, and then reload the web server.
Step 4 happens in the WordPress dashboard.