I run regression and I would to show the interaction/difference between two variables.

First, I use the ## to show the interaction. Array
Then I run the regression with the two categorical variables (the second variable is a subgroup of the first, the two group aren't disctinct group). The result shows me the difference between the two categorical variables. How explain that the value for the subgroup is the difference between the two group?
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What's the best to show the interaction and the difference between the two groups? The first regression or the second one?