How to sync multiple Apple devices

With iCloud.

Change your iCloud feature settings [iOS and Mac] - https://support.apple.com/HT207689 -

"When you turn on iCloud for an app, its information is safely stored in iCloud and automatically kept in sync across your devices. When you turn it off, the app will no longer connect with iCloud, so your data will exist only on your device. You can choose which apps on your device you'd like to use iCloud, or turn off iCloud completely."

When you turn off an iCloud feature on a device, information stored only in iCloud for that feature is no longer available on the device. If you turn off Mail, Contacts, Calendars, Notes, Reminders, or iCloud Photos on just one device, you can still view and change your information on iCloud.com and on another device you set up for that feature.

Alternatively you can do it by syncing them all to a common source which would be a computer. You can also do a compromise and sync some things with iCloud and others only with a computer. I do it that way and I manage to not to have to buy additional iCloud storage. It's a pretty flexible system.