You asked the right question.
The textbooks are wrong everywhere on this and the Phet sim is less wrong,
but it is unclear.
An article that points out this and offers a correct understanding and the
answer to the question is found here;
On a Common Mistake in the Description of the Photoelectric Effect
TPT 57, 536 (2019)
If you or anyone else does not have access to this TPT article then please
contact me.
One less common but published book that does treat this issue correctly is;
Teaching Introductory Physics
by Arnold B. Arons. (Wiley, 1997)
This article states that the necessary concept is the quantized *photon
flux* of a light source. But this concept is excluded and results in a mix
up with another related but different quantity, the light *intensity*.
- JZ
--
Gerald Zani
Senior Engineering Technician
Brown University School of Engineering
(401) 863-9571