The problem is - In a standard transportation problem, with destination's demand limited, but each factory has unlimited supply to cater to the needs of destination. How should we modify the problem in such cases. I have the following idea.
- Assign each factory the capacity of aggregate demand of destinations
- Add a dummy destination with transportation cost zero and demand equal to the unbalanced part( Aggregate demand * (no of factories - 1)
Then solve the problem.
The initial solution - One of factories will be supplying its entire supply(modified new supply) to the dummy destination.
Also, Can I use the same method with modification, in case destination can handle unlimited supply but the factories has limited capacity to supply.
I have added a sample question for better understanding of the problem.

Yes, setting the supplies to a small upper bound and introducing dummy arcs and a dummy demand node is the standard approach. Yes, you can apply the same idea with a dummy supply node if the supplies are fixed but the demands are unspecified.