Engine version is also important.
Ue5 is compromised trash where just rendering frys your motherboard even when things are turned off.
If you go back down to something like ue4.18 even a potato phone is then able to run a basic scene.
Ita mostly got to do with the internal process of the rendering pipeline - i guess one can try ue5 in forward renderign for mobile, but i believe the result would be much of the same (insane cost/drain/heat to say the least).
Native resolution on the mobile device also plays a big part to this. Even in older versions.
For instance, one of the latest galaxy phones that renders double res 2k is essentially trying to run a 2k output.
Since the engine is barely capable of running 1080p at 60fps off a proper GPU. The phone is going to obviously struggle.
The only way to know what works for sure is to benchmark as many devices as you can - and you can guarantee support for those devices, while providing users with a best guess on other devices in terms of what settings to apply.
And yes, you often end up stealing the mobile device of just about anyone you see or employ, pop it in dev mode, and test… if you care
Should be noted that 99% of app developers for mobile do not.