Thin walls and soundproofing

[Taylor D. Jenkins]

When you’re living in a multifamily and are able to hear people next door or downstairs, people will say it’s because the “walls are thin.” Is this literally due to having thin drywall? Is there not good insulation? No insulation?

How do you improve the soundproofing between units side by side and top/bottom?

I’ve read using 5/8” drywall can help between walls and on ceilings. Also using acoustic insulation and resilient channels on the ceiling. Have y’all found that these measures work?