Bitwig's official Getting Started series walks through the basics of the Unified Modulation System, showing exactly how to find and route modulators to track and device parameters. The modulator icon appears throughout the interface, and clicking it in the track header opens the modulator pane where you can browse over 40 options, from LFOs and step sequencers to more unusual signal generators.
Adding a Classic LFO is the starting point here. Clicking the modulator icon on the LFO itself enters routing mode, which highlights every available target in the interface. From there, clicking and dragging a parameter like track pan sets the modulation range directly.
Each modulator can control multiple parameters at once, and track-level modulators can reach both mixer controls and any device parameters further down the chain. Pressing Escape or clicking the routing button again exits the mode, and holding Alt temporarily disables routing without leaving it entirely.