As for PRIVATELY- ISSUED plans, you can enroll year-round. However you will need a QUALIFYING LIFE EVENT to qualify for SPECIAL ENROLLMENT through the PUBLIC marketplace or an employer.
Private health ins or COMMERCIAL health ins is coverage issued by a private company or entity - simply meaning it’s not government issued ins like the ACA, Medicaid, or Medicare.
Traditionally, PPO plans are based off of relative health and age. 19% of ACA plans have PPO networks, however if they are, they will be significantly more costly than their HMO counterparts. (Forbes)