How possible and easy is for others to break in your iCloud with another Mac or Apple phone?

You can only access an iCloud account, if you know the Apple Account and the Apple Account password.

Resetting the Apple Account password would be the first thing to do in such a case, to gain control over your Apple Account again.

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1. Change your Apple Account password. Make sure that you use a strong and unique password.
2. If you can't change your Apple Account password because it's already been changed by someone else, reset your password.
3. Go to account.apple.com to update any of personal or security information that isn't correct or that you don't recognize.
4. On account.apple.com, select Devices and remove any devices associated with your Apple Account that you don't recognize.
5. Check with your email provider and cellular carrier to make sure that you control every email address and phone number associated with your Apple Account. For example, check with your cellular carrier that SMS forwarding hasn't been set up for a phone number associated with your Apple Account. 

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copied from: If you think your Apple Account has been compromised - Apple Support

Apple doesn’t allow this.

However, any device is often subject to compromise in any close domestic relationship.

Stalking is also a crime; report it to the police.

See this article:

If you think your Apple Account has been compromised - Apple Support

See this article as well:

Stay safe guide - Apple Support

and:

Personal Safety User Guide - Apple Support

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Ingo2711 wrote:

You can only access an iCloud account, if you know the Apple Account and the Apple Account password.

Resetting the Apple Account password would be the first thing to do in such a case, to gain control over your Apple Account again.

You want to secure the individual devices – or mark any that are lost or stolen as Lost – as well.

When I log into these forums using Safari, I can do so using my local Admin password on my Mac, and Face ID on my iPhone, even though the login is to my Apple Account.