Do I get WHM, or only cPanel

Both, always. WHM is the layer above; cPanel is one account seen from inside. You get the pair, not a choice between them.

Deciding which to open

• WHM creates accounts, decides the address each answers on, sets the boundaries a group of sites lives inside, pauses them, and shows the whole estate on one screen.
• cPanel handles what is inside a single account: files, databases, mailboxes, subdomains, certificates, scheduled tasks.

A shortcut that almost never fails: if the sentence in your head contains "which site" or "every one of them", you want the upper layer.

You do not need a password per site

WHM opens any account beneath it directly. There is no reason to keep a stored list of individual logins, and no reason to make them all the same so they are easier to recall. Handing one site to a colleague also becomes safe, because you can give access to that account alone.

The screen worth learning

The account listing shows every account beside its address, its space consumption and its status. Three awkward questions collapse into one glance:

• Is an address quietly serving more accounts than you meant?
• Is anything creeping toward its space or file allowance?
• Is something still paused that you have entirely forgotten about?

On an estate assembled over several years, that list is usually where you discover the thing you did not know was wrong.

A habit that saves arguments later

Resist making estate-wide adjustments from inside one site's panel. It appears to work, which is why it keeps happening. You correct one site, the other twenty-nine keep their old behaviour, and much later somebody spends an hour discovering why exactly one account is different.

Is this a reseller account?

Functionally you have the tools a reseller has: your own accounts, your own packages, your own address allocations. What matters day to day is that the boundary between your sites is a real one rather than a folder structure.

Can I give someone access to one site only?

Yes, and that is the point of keeping sites in separate accounts. A contractor working on one site gets that account and cannot see the rest of the estate.

Where do I create a new site?

In the upper layer, because that is where the address, the allowance and the package are decided. Getting those right at creation is far cheaper than adjusting them once a domain is already pointing at the site.

Fuller treatment in WHM and Accounts, and the interface reference at docs.cpanel.net.