You should only use one account for your primary email, contacts and calendar. And you should set that account up as the default account on your smartphone for those three features.
This sounds simple enough, and it would be if you only had one account. But most people in Apple World have two: Their main email account and their Apple account.
Apple really wants you to use their things and only their things. So it can get confusing.
Say you have a Gmail account and you put it on your iPhone; you also have to sign in to your Apple account.
This is where it gets messy. You should sync mail, contacts and calendar with Gmail, but Find My iPhone, Notes, Safari, Reminders, News and Wallet with your Apple ID.
Once you sync these, go individually into mail, contacts and calendar to check that your Gmail account is the default account.
Seriously, do it now.
It is the number one problem I see as the resident tech nerd to my friends, family and workmates.
What happens is you sign in to another account, maybe you have a Yahoo account, and the default setting gets changed to that account and not Gmail.
No big deal, except now you are adding contacts to your Yahoo contacts list, and your existing Gmail list stays stagnant.
Even worse is saving contacts to your phone.
Remember the days when losing your phone meant losing phone numbers you’d never get back again?
This advice covers the majority of people. If you have multiple accounts on multiple platforms and customised calendars, sharing, etc., you need to still pay attention to what the defaults are for your iPhone, but your solutions are more complex.
What you want to have is one account to rule your world and an Apple ID to sync the less important stuff.