You need a VNC Server app for the remote computer you want to control, and a VNC Viewer app for the computer or mobile device you want to control from. You can install and license these apps in advance, or download and run them on-demand, in the way that suits you best. Or do both, in different circumstances.
VNC Server captures the desktop of the computer in real-time and sends it to VNC Viewer for display. VNC Viewer gathers your input (mouse, keyboard, or touch) and sends it for VNC Server to inject and actually achieve remote control.