By default a filter in a Looker studio report applies to:

In Looker Studio, when you apply a filter to a report, it defaults to affecting all fields within that report. This means that any data displayed across charts, tables, and scorecards will be filtered according to the conditions you set. While you can configure filters to apply only to specific charts or groups, the default behavior is report-wide. "All data sources for the report" is incorrect because a filter applies to the data from the sources, not the sources themselves, and it's about the fields within the report. "Only to a single field" is incorrect as this would require specific configuration, not the default. "There’s no such feature in Looker Studio" is incorrect as filtering is a fundamental feature.