Is there a Mac computer following my iPhone and iPad

Only is someone with a Mac knows both your Apple ID and password. Then they can login to your account and see all of your information.

But even that isn't possible if you have two factor ID enabled. Then even if they know your account information, you'd get a notice on your trusted devices that someone on an untrusted device was trying to enter your account, and should it be allowed.

No, why do you ask?