I think that, by default, the store only shows apps that have been officially made adaptable to the mobile screen.You can enable “Show Incompatible Applications” in the store Preferences to list more… some of which may actually have been built to adapt to screen size by the app developers, although not “officially” by Purism.
You can also search for any app in the PureOS repository (which includes not just mobile-adaptive apps) by using the magnifying glass/search function in the store. Some of those might work on the L5 with some slight adjustment to scale. It can be hit or miss, but I’ve found quite a few usable apps there. Some of them are demonstrated in this thread: List of Apps that fit and function well [Post them here.]
P.S. If flathub is installed, you will also see flatpaks when using the search function in the store.