A picture (schematic) can be worth 1024 words…
People usually speed-control small DC fans either using a purpose-built control input or by brute-forcing the matter by hacking up the power supply, as described in greater detail here. What’s appropriate or optimal for your situation would vary depending on the precise circumstances and objectives.
A fancy digital isolator could certainly be made to work, but if a person can achieve the same goals using a product available in equivalent form from multiple suppliers, the prospects of supply disruption are dramatically reduced.