If I have a VPN configured on my iPhone, which ip address is accessible by some other app on my phone? For example, if I use some news app, will the app's server know my true ip address or the ip assigned by the VPN?
Apple doesn’t really tell applications much about vpn since there are other ways to check if a resource on the internet is reachable, so most apps just check what they want to access is available or try to request something and then indicate waiting or failure.
It really depends how smart a programmer is if they track any or all interfaces and also match up your request to their server logs which will show your NAT address in all cases.
It also depends on the VPN if it’s app specific or system general and split tunnel exposes multiple network interfaces to each app in practice.