What does a little square under the wifi symbol mean?
Solution 1:
It’s not a corner, but it is “L”. Based on Reddit threads (here and here), the “L” indicates your Switch is using local communication.
This means your Switch is connected to the internet, but the game you are currently playing is using local connections for multiplayer. One of the Reddit threads uses Minecraft as an example.
Another example, which the other Reddit thread noted and I’ve checked myself, is Pokemon Sword and Shield. This icon will appear while connected to the internet, but your game is using local communication for trading and battling