dan.com sedo.com afternic.com ...
Ok great. Thanks very much for the advice, very useful. I have loads I wanted list and some examples below so hopefully something like Sendo will help find if they have any value then.
You can sell or list them in namepros or Sedo / afternic / Dan.
If they already have 5 million downloads, their brand is established and they don't really need a .app name because everyone knows them as [AppName]app.com and it avoids any confusion. Their users aren't going to type in .app accidentally. It's also hard to say without knowing if the app name is one word or two words. If it's two words, it's not worth getting .app. They would be better off getting their appname.com and just dropping the "app" (or redirect).
I'd say .app works best if it's one word. Two word .app means people will type in each word to see if the .app equivalent is taken (more traffic for those who have that word in .app) I prefer nouns over adjectives because nouns are very versatile. Newer apps will drop the nameapp.com in favor of name.app so they can get a premium one word .app for far less than its .com counterpart and still be able to brand their app using the .app extension. I'd highly recommend taking a look at thefutureis.app to see the scope of .app development. This is just the beginning considering .app was made available in 2018. One of the biggest hurdle for those owning premium .app domain names is the price. They are certainly not cheap to hold so you want a dictionary word that's easy to spell, widely understood (no trademark infringing) and is not too long (no longer than 7 letters). If you have such a word, then there is a chance that someone down the line might want the name.
TLDR: nameapp.com is fine if you're already an established app. If you're branding a new app, there is a .app for that.
Separate names with a comma.