Okay so small thing but your nephew didn't prove crossplay works on "every system" here, he showed you it works between a Switch and a PS5 in the same house. That part has actually been around a while for Fortnite. The bigger deal is that you two were on the same wifi and probably the same Epic account setup, which makes the whole thing feel way smoother than it does when you're playing with random people online. Try it sometime with a friend across the country on different consoles and you'll see the cracks, matchmaking takes longer and voice chat can get weird. Kid still deserves credit though, he knew his way around that menu better than most adults I know.

My buddy Marcus hit the exact same wall at the Food Lion on 5th around 8pm on a Sunday. He thought he scored big with two birds for ten bucks, got them home, and the meat was so dry it crumbled like sand. He tried drowning it in bbq sauce and still gave up halfway through. Turns out they'd been sitting under that heat lamp since like 4 in the afternoon. He started going right when they open in the morning now and says the difference is night and day, still hot and juicy instead of jerky.