If I have not purposely downloaded Transmission, and I do not use an administrator account while using my Mac or Macbook, am I safe?
Yes, you’re safe.
The whole recurring theme to Apple malware is the victim must do at least one, and usually several, very deliberate step(s) to actively disable Apple’s security. In this case, you’d have to _deliberately_ download Transmission _and_ run it in administrator mode; the former isn’t particularly stupid but you _do_ have to know what you’re doing, and the latter is just a bad idea ... and the perpetrators had to go to great lengths to take advantage of both situations happing at once, and since the breach was discovered early both the legitimate & hijacked company has already distributed a self-correcting update, and Apple has de-authorized the “signing key” required to install the malware version.
Safer yet is not using either one and only using a ‘nix machina to access the internet. *grin*