Configure the correct DNS records

Point the DNS host/domain names for which you want show websites to the IP-address of your webserver.

Configure the web server

Like all modern web servers nginx allows you to assign multiple DNS host/domain names to a single virtual server with the server_name directive.

server {
    server_name website1.example.com website1.com;
    
    location / {
      root    /var/www/virtual/website1.example.com/htdocs;
    }
}
server {
    server_name website2.example.com website2.com;

    location / {
      root    /var/www/virtual/website2.example.com/htdocs;
    }
}

Let's Ecrypt

It really depends on how you configured Let's Encrypt before but you can add additional domain/host-names to an existing Let's Encrypt certificate configuration.

To add website1.com to an existing certificate use for example:

certbot --cert-name website1.example.com -d website1.example.com -d website1.com