In your PCB view, open up the "Layers" menu. This can typically be found in the tool pane on the right side of the screen or in the settings menu.

In the "Layers"' settings, you will see options for both Top Layer and Bottom Layer.

To set your board to be single layer, you simply need to turn off the Top Layer. This is usually done by unchecking the checkbox or clicking on a toggle to turn off this layer.

Now, only the Bottom Layer should be active. All of your traces will need to be routed on this layer, and components will need to be placed according to this limitation.

Separating Power Loads: If your circuit has multiple sections that each require a well-regulated 5V supply, but each section might introduce noise into the voltage supply, then you might use one 7805 for each section. This way, the noise produced by any one section will not affect the others.

Increasing Power Capability: If your circuit's total power demand exceeds the power output capability of a single 7805, you might use multiple 7805 regulators in parallel, each feeding a part of your circuit. However, it's essential to evenly distribute the power load to each regulator since they may not share the load equally due to their slight differences in characteristics. A better approach when needing more current is to use a power regulator with a higher current rating.

Navigate to the PCB layout editor from the Schematic editor if you're not already there.

Select the appropriate layer for the hole. If you want a through-hole which spans all layers, you can select any layer.

On the right-hand side toolbox, click on the 'Hole' tool.

Click on the PCB layout where you would like to place the hole. A hole should appear at your pointer's location.

You can adjust the size of the hole by selecting it and changing its diameter property in the property inspector window.