balu you mentioned double tap to lock, for that what I said applies, there is no button in the accessibility menu for it.
What such gestures require is an accessibility service provided by a third party app, normally your chosen launcher, for that purpose.
Yes you can use the power button in the notification shade to "lockdown" but that isn't accessibility or just locking the phone. Lockdown has a specific purpose in removing the ability to use biometrics to unlock and forcing the PIN/Password method.
Be warned though that granting any accessibility service or device admin permissions to third party apps opens up huge attack surface across your device.