In most cases, that is not the way things work, but there are a few streaming companies that have incorporated control of Sonos from within their own apps, but it’s normally a limited number of types of commands. In most cases, you would use the Sonos app to control the Sonos speakers. 
 

However, you mentioned in your tags that you have an iOS device, which can ‘cast’ an AirPlay 2 signal (from most, but not all) apps to Sonos….again, you wouldn’t be grouping or anything else via that method, just play music. You can coceivably send an AirPlay 2 signal to multiple speakers, but if you do that from within the AirPlay 2 menu, you’re just increasing the number of streams that your device is sending to Sonos. Much better to send one stream, and do any grouping in the Sonos app.

Thanks.

So, I thinlk you’re saying what I wanted to hear.

Use the Sonos app to group the two speakers as a stereo pair.

From there use the iPhone’s Music app to play music.

That’s all the usage there would be. If this is so, theoretically there would be no more use of the Sonos app?

Sonos organises and then references speakers (individually, or associated such as a stereo pair or a 5.1 surround system) as rooms. If you set your two speakers as a stereo pair, they will be “Lounge” or whatever name you give them. Then send your chosen music to Lounge from your iPhone. 

Thank you both. I hope you don’t mind my not selecting one or the other as ‘Best answer’ - wish the system would let me click it for each of yours!

The problems we face in these times!

Its actually very common for contributions from several folk to go towards answering questions on the forums. 
 

You can use the Apple Music app for music and you Sonos app for everything Sonos. You can however also ad Apple Music as a source to the Sonos app. Just try and see what you like best. 

A followup: Once the speakers are setup in the Sonos iOS app, will my wife have to use the app/setup also or can she just start playing from her own iPhone’s Music.app?

When she is connected to the wifi network, she should be able to use airplay to cast audio from various streaming apps to play on Sonos.  Certain Sonos specific features, like grouping changes for example, will need to be done via voice control or the Sonos app though.

Maybe to help clarify, once you have all the speakers setup with your app, those tasks will never have to be repeated on other devices you use to control the speakers.  When your wifi opens up the Sonos app, she will see all the speakers you setup, music services added, etc, and be able to use them right away.

Much thanks, Danny! Exactly what I wanted to hear.