This is the important part to understand the difference between lets say the „European system“ vs. the „US system“:

In the US the Democrats and Republicans each encompass big tents that hold several groups that would otherwise be their own political parties.

The US negotiates the different political streams within one of the two parties before the elections. Whereas in most European countries these negotiations happen after the elections in coalition building. Have a look at scandinavian elections and you will see blocks that can be interpreted as a hybrid form.