You can access your default ad settings through your YouTube Studio by following these quick steps:
1. Sign in to YouTube Studio
2. From the left Menu, click 'Settings'
3. Select 'Upload defaults' > 'Monetization'
4. In the “Types of ads” section, select the checkbox next to each type of ad format you want to allow for monetized videos that you upload:
5. Click SAVE
All formats are selected by default. Learn more about available ad formats in YouTube Help Center.