How do I turn off location on my iPhone, for everything or just one app?

You've got granular control. Settings > Privacy & Security > Location Services. At the top is a master switch to turn ALL location off, but you probably don't want that (it breaks Maps, weather, Find My). Instead scroll down and tap each app to set its permission: Never, Ask Next Time, While Using, or Always. Set the nosy ones to Never and keep Maps on 'While Using'. Turn off 'Precise Location' per app if you want them to only get your rough area.

The per-app control is the answer, don't use the master switch. Set social and shopping apps to Never, keep Maps and weather on 'While Using'. Best of both.

Don't miss 'System Services' at the very bottom of Location Services. That's where Apple's own background location lives, and turning off the ones you don't need (like location-based ads) is worth it.

'Precise Location' off is underrated. The app still works with your approximate area but can't pinpoint you. Great for weather or news apps.