Why does my new iMac not sync events from the calendar or contacts on my iPhone?

[Jurek]

As above.
None of the events in the calendar on my iPhone are replicated on the iMac - I've manually backed up and sync'd.
Some, but not all, of the contacts on the iPhone appear in Contacts on the iMac.
iMac is a 2020 model running Big Sur.
iPhone is a (First edition) SE

[TimC]

Are you signed into iCloud on both devices?

[Jurek]

Are you signed into iCloud on both devices?

I am now.
But it has made no difference

[Jaded]

If you go into iClod on the web do you see the data?

I’ve solved this before by signing out then in again on devices.

However, currently I have a calendar that either appears twice on one device, or not at all on another. I have to take my pick. IClod can be a right pain.

[Jurek]

Turns out that on my phone, in iCloud, neither my contacts nor my calendar were checked to sync, or backed up.
Sorting itself now.
Genius Bar rocks.