How do I sign out of all devices?

https://appleid.apple.com/account/manage/section/devices

A full device list for your Apple ID is available here. For each device, tap the device and tap Remove from Account at the bottom.

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It’s much easier to sign out via the devices list in Apple ID settings.

Sign out of what?

If you are referring to a Device Sign In with an Apple ID. The best would be to change the Apple ID password.