A (gross, frankly TMI) example is that I did not know most people changed their underwear every day. I also did not know most people owned upwards of 30-40 pairs of underwear. As a child, I owned maybe 3-4 and my family washed them once a week, if that. This was when I was pretty young, but I continued doing it when I was older (until teenage years). I share this one because I think it's the most surprising/gross (?) but I just feel like I'm lagging. Sometimes, I don't even know something is wrong and it takes ages to correct a behavior.
a lot of people out there no matter which country they live in have very poor hygiene. whether it's because of toxic masculinity or just plain ignorance on how things should be kept clean. for as simple as washing your hands thoroughly with soap after peeing and/or shitting, washing all of the body thoroughly at least once a day to rid of sweat and dirt, and flossing, brushing teeth, and using mouthwash.
when i say
toxic masculinity, i mean the mentality that 'they're a man so they don't need to care about cleanliness' or something alone that line. this is pure nonsense.
in high school, used to wear my favorite shirts over and over again for the week because i loved wearing them. wondered why i would get so itchy. my classmate asked, do you wash your shirt? said 'no.' stopped doing that and the itching stopped. if anyone does this, you're breeding bacteria/fungus on the shirt and planting it back on your skin. combined with sweat and oil causing irritation.
so don't reuse underwear for more than a day. if you do, you'll be sitting on your own shit until the next time you change to a clean one. should at least have enough clean underwear to last for a whole week or until the next laundry cycle.
for women, this is far more important since they have to deal with discharge, period blood and keeping pH balance at a good level to avoid infections.
seen many, many people who doesn't or barely washes their hands after using the restroom. barely as in just letting the water flow on their hands for less than 5 seconds, then proceeds to touch every commonly touched objects such as door handles.
it's also common and a normalized behavior for someone to only use dry toilet paper to wipe their ass. you're actually barely cleaning it and mainly smudging the shit stain in between those cheeks, then it'll transfer over the underwear. the superior way is to use soap and water, but no one really does that. only know some people from a specific country (not the US) that does this. alternative is using wet wipes.
there are a lot of things that people can do to actually keep themselves clean, but most of the time it's not practiced nor cared about.
was taught a little and made aware by an ex in my 20's about hygiene and other things such as psychological concepts, but learned most on my own by experience or by intentionally looking it up. it was a massive struggle. was judged and made fun of by friends, family and strangers for my lack of knowledge on things many times in my life instead of sincerely teaching me. it didn't help that I was still experiencing culture shock. it's what made me push to learn subjects on my own.
being curious and asking questions is the key -- what, why, how. think about the things people take for granted and challenge it.
try to stay clean, or don't. it's your body in the end.