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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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.

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