The business plans are the ones built for more than one site: five, fifteen or forty websites on a single cPanel account, with 10, 15 or 20 GB of NVMe and 4 to 8 processor cores behind them. They are one account with one login and one renewal date, which is the point of them.
Adding the site
In cPanel, open Domains and add the domain. cPanel creates a document root for it under your home directory, adds it to the web server, and sets up a DNS zone if the domain is using our nameservers. The new site is live as soon as the domain resolves to the account.
Point the domain at the account first. If the domain is registered here, set its nameservers to the ones in your hosting welcome details. If it is registered elsewhere, either change the nameservers there or point an A record at the account's IP address.
The certificate
A free certificate is issued for each domain on the account once that domain resolves to it. It is not instant: the issuing process checks that the name points here, so a domain added before its DNS has moved gets its certificate when the DNS catches up rather than when you add the domain.
What the sites share, and what they do not
They share the account's disk, its processor and memory allowance, and its backups. They do not share files: each has its own document root, and nothing in one is reachable from another through the web.
Databases are per account rather than per site, so a second site gets its own database and its own database user. Keeping those separate matters: one site's database user should not be able to read another site's data.
Mailboxes work per domain, so each site can have its own addresses on its own name.
When a second account is the better answer
Sites on one account rise and fall together. They share the allowance, so a busy one takes processor from a quiet one; they share the backups, so a restore is an account-level event; and they share the login, so anybody who can administer one can administer all of them.
That is fine for sites with one owner. It is the wrong shape when the sites belong to different clients, when one of them is substantially busier than the rest, or when somebody needs access to one site and must not have access to the others. In those cases a separate plan for the separate site is cheaper than the day you wish they had been separate.