As survivor testimonies indicate, the Sonderkommando members were well aware of what was going to happen to them. The Sonderkommandos were regularly killed when they were no longer able to work productively and replenished by new transports. Their life expectancy was generally no more than a few weeks, aside from a few lucky prisoners with special skills needed by the camp staff. In some cases, the old Sonderkommando members were shot, while others were gassed. By that point they were physically and psychologically destroyed and unable to resist; unsurprisingly, suicides were quite common as well. When an extermination camp ceased operation, their final Sonderkommandos were deported to other camps and gassed there (for example, the last 300 Sonderkommando personnel from Belzec were gassed at Sobibór in June 1943).

Of course, the fact that the Sonderkommandos knew what was going to happen to them motivated some of them to fight back before it was too late. There were famous revolts by the Sonderkommandos at Sobibór and Treblinka as the operations at those camps were winding down in the fall of 1943. There was also a Sonderkommando revolt at Auschwitz in October 1944, which has been depicted a couple of times in films in semi-fictional form (The Grey Zone and Son of Saul).

My main source was Yitzhak Arad, Belzec, Sobibor, Treblinka: The Operation Reinhard Death Camps (Indiana UP, 2018). If you're interested in the subject, I'd definitely recommend reading some of the survivors' accounts, including Filip Müller's Eyewitness Auschwitz and Miklós Nyiszli's Auschwitz: A Doctor's Eyewitness Account. Claude Lanzmann's film Shoah includes interviews with a number of former Sonderkommando prisoners as well, including survivors of the revolt at Treblinka and two survivors from Chelmno.