I was hired to be an Android developer for a startup. Other engineers on my team bounce around back-end, web iOS and Android. I make sure to review every Android PR that comes in but at the same time I have strived to review code from other projects like back-end and iOS. My primary language is Kotlin so I can offer advice on that but since others are written in typescript/Swift I am not sure how to offer reviews although I want to. I mostly end up asking questions.
First, I highly recommend this Q&A from a software engineer at Microsoft covering a similar topic. There's a lot of general advice there on how you can legitimately add value reviewing code outside of your domain.
That being said, I really like your instinct around expanding your scope via more holistic code review - I think this is really important in a startup to build resiliency. It's pretty common for a startup to only have 2 iOS engineers or something, so if 1 iOS engineer leaves for whatever reason, the remaining is stuck without a code review partner. This can be fixed if non-iOS folks can do a decent job reviewing iOS code as well.
Since you're an Android engineer, I have 2 more specific pieces of advice for you.
I mostly end up asking questions.
There's absolutely nothing wrong with this - This is a huge part of effective code review! This is good signal for the author, because if they're getting way too many questions about the purpose of their commit or how their code works, that could be a sign that they can improve in some way (adding more context to summary, using more modularization, etc).