That's not how it works. Apps cannot access the traffic of other apps, let alone decrypt it. There is no way DDG Browser does what you claim it does. They do not even claim that themselves.
EDIT: Unless, of course, it works as a VPN, which apparently it does.
They're making shit up. I can assure you that the DDG app does in fact block trackers in other apps, by functioning as a VPN. Give it a try, it really does work.
There's also the PersonalDNSFilter which does the equivalent while being a tiny open-source app that serves only for that purpose and also somehow still not getting banned from the Google Play store or AdAway which also has this feature or TrackerControl or...