How a 0.5% false positive rate can lead to 70% of results being false

In the video on this check the woman saying, new tests, very low false positive rate, 0.5%, then she says 100,000 tests in Liverpool produced 700 covid positive results WITHOUT saying 500 of those must be false positives (0.5% of 100,000 = 500) only slightly higher false positive rate would make ALL results false, as it is 500 out of 700 results being false is over 70% false positives arising from a 0.5% false positive rate

She starts at 9minutes28

https://youtu.be/foFSoYbooMM