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.