Starting with your original question, if you double click on the terrace face to select it and its edges, you can then use the move tool to move it vertically down to z = 0, locking to the z direction by pressing the up arrow after starting the move or by starting and watching for the “on blue axis” inference and then pressing shift to lock that movement axis. You can inference to, for example, the bottom of a wall or to the model origin to get the desired height. The following was recorded using the desktop version, but you can do exactly the same in the Free Web version.
There are a few other issues in the model. For one, it isn’t using tags appropriately. Only groups or components should have tags assigned, all edges and faces should be untagged.
You have generally made good use of groups and components, but there are also some loose faces and edges in the model. For example, the terrace face in your question. You should make a group or component of an object as soon as you have drawn enough to capture its basic location. That will avoid any problems with things sticking together when you didn’t want them to and also makes it easier to select them as a unit to move without distorting.
You have the camera set to parallel projection. I would recommend using perspective while creating a model, both because it is more realistic and because it is less prone to clipping.
You have several terrain sources in the model, and I don’t know which you want to take as truth. But some of the buildings project below the ground surface. You might want to move them up, but because the ground slopes slightly there will always be some part that projects through or else a gap between other parts and the ground.
You also have several ground images in the model and again, I don’t know which you want to use as truth. I notice that with regard to vue_plan_cadastre the buildings are slightly shifted from what I assume are the corresponding ground footprints.
Edit: I also notice that the “pylone 20kv 3#1” object is a large distance off the ground. It probably also needs to be moved down.