Discord pays for their own infrastructure, so their “Nitro” subscription pays for additional features, which off the top of my head include:
larger attachment sizes
higher video quality when screen sharing in a call
higher audio quality in calls
using custom emojis and stickers from other “servers” (they aren’t actual self hosted servers, that’s just the name of discord communities, they have a collection of text+voice channels that you can talk in)
If you create your own server you can add whatever image you want as custom emoji/sticker. Someone probably uploaded an image of Tux as a custom emoji for their community
Discord pays for their own infrastructure, so their “Nitro” subscription pays for additional features, which off the top of my head include:
The “Linux” emoji, as the other person said, is likely from a different community (“server”)
If you create your own server you can add whatever image you want as custom emoji/sticker. Someone probably uploaded an image of Tux as a custom emoji for their community