I asked the Internet to explain how federal courts are organized.
The federal court system has three main levels: district courts (the trial court), circuit courts which are the first level of appeal, and the Supreme Court of the United States, the final level of appeal in the federal system. There are 94 district courts, 13 circuit courts, and one Supreme Court throughout the country.
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Each year, hundreds of cases are appealed to the Supreme Court. Each case goes to a justice for a quick review. Barrett opined that the students’ case would fail certiorari ( look it up. ) and stopped it. The other justices can reverse her decision.

How can Barrett alone block anything? She's a member of the Supreme Court not their Queen.