I definitely recommend using UrBling (basically masculine McBling aesthetic) and Post-Grunge Maximalism as jumping off points for aesthetics! Granted, dudes had plenty of different styles, but those should give you an idea. Also "2000s older brother core" is kind of a grab bag of the vibe too. For music, some of the big stuff was nu-metal, rap-rock, hip-hop, post-punk, and stuff like that. It might sound cheesy, but game soundtracks from the late 90s/early 00s are a good place to get an idea of the vibes. Games like Tony Hawk's Pro Skater 2/3, Need for Speed Underground/Underground 2/Most Wanted, and even the Madden or 2K Sports games have lots of songs by bands that were popular with dudes at the time. Video games were HUGE with dudes in that era, and it was still kind of the time where gaming was really a male space still. Also in general with clothes, unless you're going for more of a 2006 era emo/scene style, it was all about the baggy jeans, skate/surf brands, huge skate shoes, caps, beanies, chains, hoodies, that sorta thing. Hope that helps!