For merge block see reference: https://www.mathworks.com/help/simulink/slref/merge.html
However, if you try to model as merge_block.jpeg you will get a simulink error because the inputs do not have same dimensions. Merge block does not "merge" dimensions.
Merge block is generally used when you use for example an if action subsystem where depends on an if condition, an if action subsystem is active.
Example:
In above example, when sine wave is greater than zero, then output of merge block is 1, otherwise is 0.
Inputs of merge block have same dimension and data type and the driving signal is the most recently computed. Similar example is by using enable subsystem.
