You can run functional tests individually or you can run the test runner which will run all the functional tests.
From Jon Atack’s doc on how to compile Bitcoin Core and run the tests:
test/functional/test_runner.pyto run the standard test suite (trytest/functional/test_runner.py -j 60or a similar high number to run the tests more quickly in parallel)
test/functional/.pyto run an individual test file
test/functional/test_runner.py --extendedto run the extended test suite
test/functional/test_runner.py --helpto see the various options for running tests
