Just saw this on GameFAQS. Some paragraphs:
Ever since it was first announced that a majority of the Switch's online services would be delivered via a companion app - key features to online gaming such as voice chat - there have been some concerns about how exactly it'd all work. And I'm sorry to report that, right now, it's a convoluted mess.
Splatoon 2 is currently the only game to support the app, and here's how you go about setting up a room for you and your friends. First, you'll need to head to the Online Lounge in Splatoon 2's multiplayer area. From there, you'll be prompted to create a room, and you can push a notification out to your phone. It's on your phone - and only on your phone, it would seem - that you can then invite friends into a room.
It's something of a faff, and it doesn't look like it'll be worth it - at present, you can only take part in private battles, and the voice chat itself seems extremely limited. Unlike with Skype, if you flick aside to another app on your phone, you'll lose the ability to talk to your friend. So, if you need to head to an IM client as I did, to figure out with a friend exactly how to make sense of it all, you're fresh out of luck.
You'll also need your phone screen on for that voice chat to remain connected - meaning it guzzles battery in the process. It's an almost aggressively limited service, and this is before we get into the mess of cables you'll need if you're going to want to play online with headphones in the Switch's handheld mode - something which it would seem is only possible with a separate splitter available elsewhere.
