No. That's not how it works. Again, the preinstalled apps only ever compete with other preinstalled. You cannot use that space for regular play store installed apps. Ever. The area isn't writable and has a size that is fixed based on the chipset that is being used.

Also, as someone that has actually used a 16gb 6s recently... You're just dead wrong on how much apple use up by the system. Out of those 16, less than 1g was actually available for use, with every single optional app removed. System itself takes up about 8 and "Other" takes up the rest.

Also, if you're running a 12g android on a 16g phone... Stop using chinese copies that are breaking licenses. 12G means it's full android, which isn't allowed on 16G phones and hasn't been for a long time. These days, 64G minimum for full android. Below that and it should be Android Go which is a stripped down version specifically for low end phones. It only uses about 5G for the system partition (where bundled apps go) as a maximum, though most roms will be even less.