The tag won't be 0, 0, 0 unless you program it to be. You can set it to any values, and you can have multiple tags in the array (see here https://docs.modalai.com/voxl-vision-px4-apriltag-relocalization/#configuration-file)
Yes, you can put a tag at say (100, 100, 0). You can fly to say (97, 100, -3), and the vehicle can see the tag and then 'infer' the tag is at exactly (100, 100, 0), and you can have the drone then fly to (100, 100, -0.5) and then land, for example.