Can I use passkeys locally on an iMac without iCloud?

Yes, you can use passkeys locally on your iMac without iCloud. iCloud Keychain is primarily for syncing passkeys across multiple Apple devices, not a requirement for using them on a single Mac.

On macOS Sonoma, passkeys are stored in the local Keychain Access system if you’re not using iCloud Keychain. That means as long as you create or accept a passkey on that specific iMac, you can use it locally on that same machine, even if you don't have an iPhone or another Apple device. When you log in to a site that supports passkeys, Safari will prompt you to use the built-in authentication method — typically Touch ID, or your user password if Touch ID isn't available — and it will store the passkey on that Mac.

So in your case, if you set up the passkey on your iMac while logged into your user account, Safari will store it in your local login keychain, and you'll be able to authenticate when visiting that website again from the same iMac. There's no requirement for a smartphone, iCloud, or multiple devices in this setup — just be aware that if the iMac crashes or is replaced, and you're not syncing with iCloud, you'd need to set up the passkey again (or fall back to another login method if the site allows).

Re: “… And, (I hope lastly) will I get to choose which sites I use a Passkey …”

Passkeys are unique to you AND the site.

i.e. Your access to each individual site will be authenticated using a separate passkey unique to that site.

This is part of a passkey’s phishing resistance.

Example: Your passkey to a legitimate xyx.com will not work at a fraudulent xyż.com …

… nor will it reveal any of your private credentials to the fraudulent site.