That’s optional, and opt-in -- it asks you during phone setup whether to enable Siri.

It does try to nudge you into enabling it, but if someone kept pressing that button and invoking Siri accidentally, they could open Settings and search for “siri” to disable it, which I think is reasonably discoverable.

Bixby is also optional. You can disable the key trigger from the settings, exactly like iphone.

Depends what phone. S8 for example had a Bixby button that could not be disabled.

On my S10, there is a seperate Bixby button on the left side, so it's not shared with the unlock button. The best thing about the Bixby button is that I can freely remap it with third party applications, meaning I have a dedicated, customizable, physical button, something all phones should have.