Bathtub water drain moving toilet water
All these lines are connected, but it's not easy to say how without inspecting under the floor. I'd say there's a good chance the shower, possibly sink too, are connected to a one vent, and that drain is a wet vent for the toilet where they join further down stream. Then everything runs down to a main stack. That would be a fairly typical setup.

If you see bubbling in the toilet from the bath, or hear gurgling in the tub from the toilet, you may have a problem with a blocked vent. Otherwise, some movement can be expected. As the water rushes past the wye connection to the toilet, it causes some pressure changes pushing the water up or pulling it down on the other side of the trap (the toilet bowl). You're also likely to see that on a windy day just from wind blowing over the vent pipe on the roof. Other traps do the same thing, but you can't as easily see them.