1. Don't install any proprietary apps - open source only. 99% of these problems and ads go away. 2. Use an ad blocker in a browser that lets you use one. 3. Do not allow any notification except literally urgent things like a phone call or message from your partner.
If you do these things then the phone once again becomes a tool that serves you.
Once when I was trying to join a pickleball class they required an app to sign up. There was no web, email, phone, or paper based way to do the thing. I emailed them to suggest they use a ticketing system that does not require the app because many people don't want to install an app, and then I signed up for pickleball at a different club that didn't require an app.
One of our bank accounts has an authenticator which is a proprietary app. I run that on an old tablet so I don't have to run it on my phone.
This was inconvenient and I do understand that this is happening to people more and more. There is often a less convenient option to do the thing though, and I think it's good to try to use the less convenient option and tell organisations that you don't want to use an app. Basic resistance 101. Maybe people have forgotten how to be inconvenienced for the sake of principles.
Denmark and it's laws sound particularly difficult.
Edit: I realise it is a privilege to be able to take the inconvenient option and resist installing apps. If you are not able to resist because of your circumstances, I will continue to resist for you.