To be clear I wouldn’t delete my banking app if it showed me a marketing notification, I would go into the app and turn off that kind of notification. I have notifications turned on for my banking app because of 2FA and transaction notifications. In the past I was actually able to stop a fraudulent transaction within minutes because of a transaction notification so I value them.
Unfortunately, companies know most people aren’t like us so they keep pumping out the crap.
You can't if it's a junk app. Many reputable apps today have properly described notification channels and you can turn them off selectively (under android at least). That's what I did for my banking and several other apps. Every "marketing" channel was turned off.
Alas it seems like few apps have it implemented. Hiss boo. Wish this was available of Web Push.