UK; it varies, a lot. Schools may run fundraisers for specific purposes, or just as an annual event.
I’ve helped out a friend’s kid’s school a few times doing face painting at their annual one. They had a bunch of little stalls, some run by kids/parents/random friends who got dragged along, and a few pro stalls which presumably paid to be there. Raffles, games, lucky dips, that sort of thing. A faintly unnerving amount of alcohol as prizes.
That’s at primary level. I know some schools rent out halls and gyms over holidays and even weekends, because I’ve been to events in a few. They’re generally quite cheap, and especially for smaller towns, it seems like a good plan to me, as there often aren’t many- or any- other budget venues for community events.
Aside from that, I mainly know what my old school did, because I don’t have friends with kids at secondary level yet.
Parents paid for some school trips, though there was at my school, frequently an emergency fund if just a few kids can’t afford it (for stuff like museum visits where the whole class was going, not for things like the trip to France). They offered stuff like out of school clubs (mostly free, but some had a nominal charge) and 1-to-1 lessons on instruments at an extra fee. Never heard of charges for sports teams, aside from bringing your own kit.
For school concerts, they charged entry (£2 or so, not a large sum), and sold drinks and cakes.
I never brought home any letters straight up asking for money, aside from for trips.
My brother’s school, however, sent a lot of letters asking for money, which I believe my parents mostly grumblingly paid.
According to my brother, boys (it being a very old school for boys) with parents who didn’t cough up got harassed by some of the teachers, and even got detention for not paying some of these ‘totally optional’ fees, and you couldn’t be a prefect if your parents didn’t pay. It was widely agreed to be ridiculous, and something the school only got away with due to its age and reputation.
There was one annual ‘totally optional’ fee for, iirc, £100 for ‘laboratory equipment’, which supposedly was for replacing stuff the student broke. I remember him telling me about that one 'cos one of his classmates, having not broken anything all year, decided he was going to finish up the year by helping himself to £100 worth of equipment, mostly in the form of the batteries out absolutely anything that had batteries, and a few small, easily overlooked but vital bits from various expensive devices.
They still send my brother letters begging for money now, nearly 20 years later.