Can I transfer my data from iPhone 11 to iPhone 6 Plus? (Older iOS Version)

I’ve forgotten: Does iOS 12 allow access to iCloud? If it does, you can sync data to iCloud from your 11 and then sync it to your 6 Plus from iCloud.

Contacts and Calendar sync to email servers, so if you have the same email account(s) on both phones those should sync on both.

Then you can definitely use iCloud to sync data. Go to Settings/[your name]/iCloud on both devices and turn on the switches for data you want to sync. If it is more than 5 GB you will have to “rent” additional iCloud space; it starts at 99¢ a month for 50 GB.

You can’t do it. You can only transfer to a device with the same or newer version of iOS.