It's transparent to the apps, it's included in all GPS (or technically GNSS) locations. GPSTest has a built in accuracy test mode where to can measure your phones location accuracy. Read more about that here: https://barbeau.medium.com/measuring-gnss-accuracy-on-androi...

I noticed a huge difference when I switched from my older phone to my current with dual frequency, and I usually log runs, walks, and bike rides so it's easy to compare the tracks. But maybe my old phone was just bad, my Garmin watch uses single frequency and it's usually quite good anyway. I've seen a few reviews of new Garmin watches and there dual frequency seems to make a small and not always noticeable difference.

They might not notice a huge difference depending on where they use location services. The real accuracy advantage of dual-frequency GPS only become apparent in degraded signal environments, such as next to tall buildings (multipath signal reflections) and under tree canopies. With a clear view of the sky, regular GPS is already quite accurate (especially if the device also supports other GNSS constellations).