On the left side of the Blocks Editor screen, we have two columns of blocks. Left-click on the column tab—Built-In or My Blocks—to reveal the headings (we call them "drawers" in each of those two categories (see the following screenshot for location):
Starting a new app in Designer then opening the Blocks Editor and clicking on the My Blocks tab shows us that there is very little in it initially. The My Definitions drawer is empty because we have not defined anything yet (but will in this section). The Screen1 has a number of blocks relating to screen control in it because the Screen1 component exists by default. Currently, only one Screen component per app is possible.
One quick note about the statement, "only one Screen component per app is possible". Those of you with a little programming experience are already screaming (and rightly so) "you can't do anything with just one screen!" Well, not to worry—we can have as many virtual screens as we like, and I'll show you...