So imagine you go to tiktok.com and you click on a link to bestbuy.com/cool-product-i-want-to-buy. But instead of taking you directly to bestbuy.com/cool-product-i-want-to-buy, it keeps you on tiktok.com and just opens an iframe with a keylogger injected into it.
So then when you enter credit card info into the bestbuy.com UI, the tiktok.com JS can see what you typed.
(This scenario is largely impossible these days, due to modern browser security.)
The difference is that if you witnessed this kind of XFS in your desktop browser, you might notice it because the location bar still says tiktok.com, because you never actually left the site. But in a mobile in-app browser, you don’t need an iframe. You can inject JS directly into the browser itself, making it invisible to the user. As far as you can tell, you’re on regular ol’ bestbuy.com, not a modified version of it.