CorelCAD follows CAD standard concepts for selecting objects.

Take a look at this tutorial video for being introduced to standard selection methods and options.

...if "multiple-selection mode" with adding objects to an existing selection when clicking onto them isn't your preferred way, you can change that to a "Replace selection" mode in CorelCAD.
That will be the preferred option especially for users that are more familiar with other graphics tools (such as CorelDRAW or Corel DESIGNER).

The Properties palette (Tools > Properties...) hosts the option for specifying the selection mode at the top of the docker: The button next to the dropdown list of currently selected objects toggles between the (default) "Add to selection" mode and the "Replace selection" mode:

Default "Add to Selection set" mode

Alternative "Replace Selection set" mode

When using the "Replace selection" mode you will create a new selection everytime you pick an object with your mouse button.
However, you can still add objects to the selection by holding down the Shift key when selecting other object(s).

Tip: You can also activate a "click and drag" mode for getting a similar mouse behavior for moving objects as known from other graphics applications.
Through Tools > Options... you can reach the "User Preferences" tab with the "Mouse Options".
Click "Enable Click and drag" to move objects on screen by holding down the mouse button on one or multiple selected objects: