Would an iPhone isolated to only financials be a more secure way to go after critiquing set up?

My mom wants to secure financial/banking. Shes asking me questions I don’t have answers to. But she is alarmed with all the data breeches and most recent cell companies. She wants to isolate all of financial dealings on 1-phone with nothing else and to delete any association (apps, auto user name, password autofill) from her and my dads phones. Leaving the “financial phone” at home always.

She is wanting to buy a used iphone from a credible source and has …

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