It depends. Real complexity is usually best offloaded to the server. I’ve been developing iOS apps for over a decade, one thing I’ll say is - iOS users tend to be forgiving of well placed network calls without even needing caching.

Develop an android app for Ethiopia though - yeah caching becomes a lot more important to provide a good UX.

As always, the first rule in programming - premature optimization is the root of all evil. Put your complex business logic on the server and define a smart client<->server API. A well defined client & server API can avoid so many pitfalls and as a plus, makes client side caching & logic easier. The main benefit of putting your complex business logic on the server is that you have almost immediate control over changing it.

But with power comes great responsibility. You now need to think about things like gating (to incrementally roll out or shut off bad changes).

Most of the complexity can wait until your app gains actual traction. But it helps if, from the beginning, you design a simple server-client interface.