Can my employer access my iCloud data if I sync it on my managed Mac
With a managed Mac where &quot;everything is controlled by the employer&quot;, the employer could technically access your personal data stored on that Mac - yes. It doesn't matter if the data comes from iCloud or other places.
Note that your employer probably does not access your personal data, and depending on where you live and what you agreements are, it is probably not legal for them to do so. However, it is very probable that they could access the data given the necessary intent and technical knowledge.
My advice would be not to store any personal data on a company laptop, unless you wouldn't mind your employer or a system administration to stumble upon that data.