Boagworld has a great video on his setup and how it all works: https://boagworld.com/working-in-web/omnifocus-2/
Getting things done book: https://www.amazon.com/Getting-Things-Done-Stress-Free-Produ...
OmniFocus: https://www.omnigroup.com/omnifocus
Technically they could easily sync via iCloud and encrypt the stored data, but of course for financial reasons companies prefer to be able to access and sell all your data...
I think the reason a lot of companies don't use iCloud is because some of the various ways in which iCloud worked have not always been reliable or suited to the task at hand.