Initial enrollment period is the time when you are completely eligible to enroll in Medicare. This time starts when you are 3months away from your birth month and it ends 3 months after your birth month.
These 7 months you are perfectly applicable for any plan of medicare.
For example, Jack Allison turned 65 on june 2021. So as per rules he can enroll himself between 1 march 2021 to 31 oct 2021.