Two iPhones, two different phone numbers, one Apple ID: how do different apps that require login react

I'm thinking on getting a second iPhone as I already have two separate phone numbers in use. So far I've used both numbers in one iPhone (SIM + eSIM), but now I've started to think that separate phones would be better idea.

Before rushing into this I'd like to know if anyone has got similar situation of using one Apple ID in two different phones, and how do different apps work in a situation like this, specially those apps that have accounts created and need login in to those accounts to use the application. I've regocnized two possible problems so far, and my questions are:

Do applications log you out on the other phone if you log in to that app with the second phone (meaning that I'd need to once again log in to that app when I switch phone again)?

If I have an application on both phones but I've created separate accounts to that application to be used with that particular phone, can I still use iCloud as a backup, or does the situation get the backup confused and mix the separate information coming from that app with two different accounts?

Mostly I'm interested if I'll run into problems with bank / payment applications and also if it's possible to have separate accounts for WhatsApp, for example. My guess is that everything should work out just fine as most of the apps can be used on multiple devices anyway, and usually they do separate the device they're been used on, so there shouldn't be that many issues.