What are assumptions in JUnit 6 and how do they differ from assertions?

import static org.junit.jupiter.api.Assumptions.*;

class AssumptionsDemo {

    @Test
    void runOnlyOnLinux() {
        assumeTrue(System.getProperty("os.name").toLowerCase().contains("linux"),
            "Skipped: not running on Linux");
        // Test body: only executes on Linux
        // On Windows/macOS: test is ABORTED (not failed)
    }

    @Test
    void runOnlyInCiEnvironment() {
        // assumingThat: execute body only if condition true; continue otherwise
        assumingThat(
            "CI".equals(System.getenv("ENV")),
            () -> {
                // This block runs only in CI
                verifyDeploymentHealth();
            }
        );
        // This line always runs regardless of the assumption
        assertEquals(1, 1);
    }

    @Test
    void skipOnNullDatabase() {
        assumeFalse(System.getenv("DB_URL") == null,
            "Skipped: DB_URL not configured");
        // Integration test body
    }
}