(b) If an application last updated with updates on or after January 1, 2026, was downloaded to a device before January 1, 2027, and the developer has not requested a signal with respect to the user of the device on which the application was downloaded, the developer shall request a signal from a covered application store with respect to that user before July 1, 2027.
(f) “Developer” means a person that owns, maintains, or controls an application.
1798.503. (a) A person that violates this title shall be subject to an injunction and liable for a civil penalty of not more than two thousand five hundred dollars ($2,500) per affected child for each negligent violation
So a developer of a FOSS application that gets installed on a device on California via a 3rd party app store (maybe F-droid) must have implemented a query to the OS for this data. Even if the app does not actually provide any inappropriate content or actually any content.
Nor does it matter if he is involved in the distribution of the app to California, a FOSS app redistributed via a 3rd party (F-droid maybe) would make the developer subject to this.