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The iPhone 6 Plus cannot update beyond iOS 12.5.6. It is too old and lacks the hardware to run anything newer.
Go to Settings->General->Software Update to download iOS 12.5.6
If the apps you want require a higher iOS version than iOS 12.5.6 then you will not be able to download them.
If you have downloaded them before on the Apple ID being used by the iPhone, you may be able to get a compatible version from your Purchase / Download history. Follow the steps here: link ➡️ Redownload apps and games from Apple - Apple Support
If you have never downloaded this apps before, then there is no way to get them now on such an old device.