Out of curiosity, when you open xCode and you want to run your app, you must to click "Play" button:
But when you hover this section, a little arrow appears (downright)
Does anybody know the meaning of that element, I thought it was a hidden menu.
When you click and hold on that button, a Menu will appear where you can switch the default action for this button.
You can see the other options here:
Note that you can modify these options by holding either shift (just build), alt (brings up the scheme selector for that scheme first) or ctrl (don't build, just run).