What do the pilots do if the plane they are operating does not have an anti-ice system?
If an aircraft is not equipped with anti-ice or de-icing systems, than the answer is simple: You don't fly into icing conditions. This is usually listed as an operating limitation in the POH/AFM/FCOM of the respective aircraft. For a Cessna 172, you can e.g. find:

KINDS OF OPERATIONS LIMITS
The Cessna 172S Nav III airplane is approved for day and night, VFR
and IFR operations. Flight into known icing conditions is prohibited.

(C172 POH, Section 2 Operating Limitations)
If you are approaching icing conditions, there are usually two options to avoid flying into icing conditions:

Reduce the altitude: The temperature is higher at lower altitudes. If you can reduce your altitude enough to raise the temperature above 10°C, you are out of icing conditions.

Change course: If you see a cloud ahead and the temperature is below 10°C, you cannot enter the cloud because that would mean you enter icing conditions. You can however deviate from your current course and fly around the cloud.