Unity VR UI canvas popup

This is more of a general Unity question than something Oculus-specific. I don’t know what interaction system you’re using, but you could look up how to detect interactions on an object with your system of choice. You could have an initially-deactivated canvas activate when the user interacts with the object. 

As jtriveri mentions, this is just a Unity dev issue. Not really related to XR. Your UI canvas should be in World Space and scaled appropriately but beyond that you're just dealing with enabling a gameObject when the trigger button is pressed. There's a number of ways to do this depending on whether you use the new or old input system, as well as whether you're on OpenXR or OculusXR.

If you're using the new input system and OpenXR this video (and the one it links to) might help with getting inputs. https://www.youtube.com/watch?v=18_JMPKAlTE

If you're using the Meta XR Core SDK (i.e. have access to all Meta's helper scripts and prefabs) then this tutorial will walk through the process of getting button input:
https://developer.oculus.com/documentation/unity/unity-tutorial-basic-controller-input/

Note that the trigger isn't like one of the other buttons (A/B/X/Y) which have Down / Up states:
e.g.

// If user has just released Button A of right controller in this frame
        if (OVRInput.GetUp(OVRInput.Button.One))
        {
            // Do something
        }

Instead, the trigger gives you a float which you need to evaluate:
e.g.

// While user holds the left hand trigger
        if (OVRInput.Get(OVRInput.Axis1D.PrimaryHandTrigger) > 0.0f)
        {
            //Do something
        }

 You'll probably not want to constantly evaluate that float, but that's something you can google and learn more about on your own 🙂