Can the screen protector film only mask banking apps and notifications, but not the lock screen and homescreen?

It's a real, albeit minor, quality of life improvement for the phone screen to only become less-vibrant for a subset of content.

The Samsung masking tech can selectively mask parts of the screen. And it has configurability so it automatically is turned on only in certain apps and things like that. Because it dims the screen slightly (when you look at it straight on), just like a screen protector, it’s really useful that it can be customized in these ways. You get to keep the quality of the screen high to a much greater amount while still keeping your privacy.

It’s clear that you haven’t really looked much into this. It’s per pixel.

Screen protectors also degrade light output