Criteria used to create and select leaf nodes in sklearn

I just want to know the details of what (and how) is the criteria used by sklearn.tree.DecisionTreeClassifier to create leaf nodes. I know that the parameters criterion{“gini”, “entropy”}, default=”gini” and splitter{“best”, “random”}, default=”best” are used to split nodes. However, I could not find more information about the threshold used for spliting.

There are some methods involved in the creation of leaf nodes: post-pruning (cutting back the tree after a tree has been built) and pre-pruning (preventing overfitting by trying and stopping the tree-building process early). It would be very useful to know more details about the criteria used for splitting to have a better understanding and be able to customize these models even more.