This bit from Bitwig's official Getting Started series shows how modulators on a parent device can reach inside nested FX slots to control the parameters of the devices within them. The example uses a Frequency Shifter nested inside a synth's FX slot, modulated by the synth's own filter envelope.
The Inspector Panel is where all active modulation routings live. From there, each routing can be moved, copied, edited, bypassed, deleted, or scaled using another modulator as a multiplier.
Saving the parent device as a preset captures everything: the device itself, its modulators, and all nested FX with their routings intact.