I cannot speak for other email providers, but my understanding for the past 10 years: this is a Google thing vs. Apple. Google both allows and ignores periods in emails. first.last@gmail.com is the SAME email address as firstlast@gmail.com you CANNOT log in with both accounts. If you created your account as first.last@gmail.com, you can only log in by using that email address.

For more information from Google on this topic: https://support.google.com/mail/answer/7436150?hl=en