Sorry for the confusion. Ads may appear in your videos even if your channel is not monetizing yet if your video contains content to which you don’t own all necessary rights. The owner has the right to place ads in the affected video. More info here: https://yt.be/help/4zdt
Thanks for reaching out–to begin your application, follow these steps: select your account icon > Creator Studio> in the left menu, select Channel > Status and features> under "Monetization," click Enable> follow the on-screen steps. Hope this helps