They simply use the Chrome app URL scheme instead of the generic URL. Similar to how you go to an app’s website and it redirects you to the app itself. There’s an app called Opener [1] that uses this and you can do similar with the Shortcuts app.

Other apps can take advantage of this too. Apps like Reddit and Outlook have similar settings to do the same. They even offer options for other browsers like Firefox. But, developers would need to add each browser option themselves, instead of relying on a defaults system from Apple.

1: https://apps.apple.com/us/app/opener-open-links-in-apps/id98...

I believe the app itself (Gmail) is requesting to use Chrome.

So each individual app would need to request Chrome, something I'm sure Google's apps can do, but [insert random app here] won't.