How to apply filters on every page in a report Looker Studio ?
First create your filter :

• Go to Resource -> Manage Filter -> Click to Add Filter and configure one.

Then apply it to the entire report :

• Go to File -> Report settings.
• On the Filter section, click Add Filter and select the one you just created.
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Bonus : You can prevent a specific component from inheriting filters from the page or report by checking the Inherit filter option in the filter configuration of a component.

In video :

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Check this tutorial to know how to apply filter on a page level instead of report !