is background app refresh something i need to worry about?

On Android there isn't one switch, it's per app. Settings > Apps > pick the app > Battery, and set it to Restricted if you don't want it doing anything in the background. Same idea, just more tapping.

not really. it just lets apps fetch new stuff while you're not using them so it's ready when you open them. costs a bit of battery and data, nothing to do with security. Settings > General > Background App Refresh, you can set it to wifi only or switch it off app by app. I leave it on for messaging and podcasts and kill it for games and shopping apps. Low Power Mode turns it off anyway.