Universal iBooks sync problems

[Big Stevie]

I'm having problems getting iBooks to sync between my iPhone 12 and my new MacBook Air 2020 that I have recently purchased.

On my iPhone I have the iBooks app and have some audiobooks in there. But when opening iBooks app on my newly purchased MacBook the books don't appear.

How do I check my settings are correct for content to sync between devices? I want both devices to contain all my iBook content.
Thanks.

 

[Big Stevie]

I'm thinking now that perhaps they aren't even supposed to sync, maybe you just have to manually download them on each device?