You can move variables within a model, as well as between models.
Within a model
Variables can be moved within a model by simply dragging/dropping. You can move multiple variables at once by multi-selecting the variables (cmd/ctrl + click, or shift+↑/↓) and dragging/dropping.
If models are linked together, you can easily move/copy variables (one or multiple) from one model to another by right-clicking, and hitting Move/Copy variable, and selecting the destination model. You can think of Move like Cut/Paste, and Copy like Copy/Paste