It is assumed you have a device. Not installing isn't a reasonable option. You may think "just choose different banks, jobs and cards" but suitable alternatives were not available.
Having a second device would be ok, but very inconvenient and a little expensive due to needing multiple mobile contracts.
If those apps start to include more user hostile features such as spam and data collection, if there are no suitable alternatives we will live with those features. To some extent I am already seeing spam in bank apps, in the form of interstitials advertising shady third party services.
At the moment the app store policies and human review offer some buffer against those features being pervasively standard for such apps, but it's not hard to imagine a dystopian future where all e.g. BankId apps that are required to make payments for food everywhere all seem to have built-in spamvertising and tracking functions that no technical measures like sandboxing can prevent.
By then the app could've siphoned all of your data.
If you think Apple's vetting process is adequate protection, exclusively use their App Store.
Websites can also siphon your data, yet Safari let's me browse the websites unfiltered.
A centralized monopolistic app store isn't solving the same "problem."
There have been numerous cases of Apple missing malware in the App Store, here's one: https://www.wired.com/story/apple-app-store-malware-click-fr...
Desktop has much better tooling than it used to, including code signing, built-in browser scam website filtering, and automatic malware removal tools distributed by OS vendors. The late 90's was full of "take over your computer, time to wipe your hard drive" level viruses and malware (and I downloaded many of them!)
Macs even have recently added sandboxing for manually installed apps similar to the ones seen on App Store apps, such as OS prompts asked to allow access to personal folders, camera, desktop sharing, and other sensitive areas. Even a manually installed app is less likely to access private things because of this addition.
If AltAdBrowser (which, I assume is spyware/adware) is our level of harmful software we're talking about, it should be noted that app stores allow this type of data collection with user consent. Just check out the privacy tab in Apple's App Store for Facebook or TikTok.
Just like on a PC, all it takes is one uninformed download button press to install spyware/adware on your iPhone.
Your relative could also just only click download buttons within the app store on their desktop. That would be a very easy piece of advice.