I'm not talking about technological restrictions; I'm well aware of the pixel counts and color depth of modern displays. They do not get rid of the design constraints of trying to make a usable interface with very high information density on a small screen.

A game like Unciv can't take the usual mobile UI cop-out of just hiding all the complexity behind a hamburger menu. It needs a large number of individually recognizable UI elements that are all significantly smaller than a thumbprint, because there are several pieces of information that need to be conveyed about each map tile the user can interact with. So a lot of the UI needs to be composed of icons or graphics designed under similar constraints. (And no, the fundamental design constraints and goals of icons are not the same as the file format limitations of early Windows .ICO files.)

Mobile games like Genshin Impact have comparatively little game state information and fewer discrete indicators that needs to be on-screen, so they can go all-out on the scenery and decorative visual effects. Unciv on a phone at its most zoomed-in has about as much physical screen area per map tile as Civ 5 on a laptop at its most zoomed-out; at normal playable zoom levels their respective UIs are working with very different amounts of screen real estate.