Hi @Anonymous,
Do you want to expand all the columns within the nested table?
Could you please tell me that if there is only one column existing in the nested columns?
I have a test on my side, please check as below:
ClearCollect(UnGroupCol,ShowColumns(AddColumns(nestedCollection,"LevelG",Level.Title2,"CategoryG",Category.Category2),"Title","LevelG","CategoryG"))
I create a collection like yours:

Then you just need to use the AddColumns to get the value within the nested columns:
ClearCollect(UnGroupCol,ShowColumns(AddColumns(nestedCollection,"LevelG",Level.Title2,"CategoryG",Category.Category2),"Title","LevelG","CategoryG"))
