Spam folder

Your Junk folder is the Apple version of a Spam folder; you tweak it in Mail Preferences Junk Mail tab.

All our devices have no Spam folder and we all use Apple mail. I think "Spam" is relative to the system you're using (Comcast, outlook, gMail, yahoo or other) and not Apple's mail app specifically.

We have Yahoo, Comcast and iCloud accounts only with no "Spam" folder.

I would imagine your iMac is not getting your spam because its filtering is better perhaps? I wish I had that problem ... less junk.

When you identify SPAM email on your Mac via Mail rules, you have the option of moving it to one of the Trash, or Junk folders. There are menu items on the Mail Mailbox menu for emptying those folders.