Thoughts on international diversification for long-term wealth building?

I’ve been focusing mostly on domestic investments so far, but I keep hearing how crucial it is to diversify internationally when you’re in it for the long haul. How much of a difference has it made for those of you who’ve been investing for years? And what strategies have worked best ETFs, mutual funds, or something else entirely? I’d love to hear your experiences and any pitfalls to watch out for.