You can set the default boolean operation and display mode for new boolean input objects per node using the Boolean stack or universally using the Boolean Operations options.
To set defaults for new boolean input objects per node
The defaults are set for the selected node.
This method sets defaults at the node level, giving you the option to use different defaults for different boolean nodes.
To set defaults for new boolean input objects universally
The defaults are set universally across all Boolean operations and are applied to all boolean nodes in your scene.