How do I download apps on my iPhone 6 when I have iOS 12 and no apps

it's 100% the choice of the app developers choose to support

How can I get an older version of an app … - Apple Community

and or

How to contact an app developer - Apple Support

You cannot download such apps onto your iPhone 6. It is obsolete and cannot be updated beyond iOS 12.5.5.

If you need to use apps or features that require iOS 13 or later, then you must purchase a newer iPhone.