Thank you so much for the response. I actually ended up with drawing two solids and put them together.
I am planning to use Ansys Mechanical to do the thermal analysis with partially heat generation.
The only data I am provided is the size of the part (I have drew it as a separate solid) does the self heat generation and the total power of heat it generates. I am not sure whether Ansys Mechanical has the option for me to set up the power or there is a better way to do it.
I see some tutorials online also use Ansys Discovery to run thermal simulation. Do you have suggestions which one is better between Mechanical and Discovery? (In my case, I have several different materials contacting each other tightly shown below. No gaps in between.)
The part labeled in red is the heat generating part.