DKGroup

Here is a really simple explanation of what a layer mask is. It is 8 years old, and uses an old version of Photoshop. But, it gives you a good explanation for just what a mask is and why it is preferable to just erasing pixels. Also, remember that there will be differences between Photoshop and Affinity Photo – the Layers panel is laid out differently, showing and hiding masks works slightly differently, etc. But the basics are there.
Now, here is an Affinity Photo tutorial. It's one of the "legacy" tutorials (the ones that were available prior to version 1.7). It, too, is a pretty basic overview, but it gives some pretty good examples of masks applied to Pixel and/or Image layers, Adjustment layers, and Live Filter layers.
https://player.vimeo.com/video/130972598

First things first - your result is really nice.
Using Erase to get rid of the portions of the flag that don’t overlap the sky is a fine solution, but it is “destructive.” That is, those pixels are gone forever. Using a mask is a “non-destructive” way of doing the same thing; the difference is that all of the flag layer is still there, but part of it is hidden.
Your solution certainly works (and, as noted, your result speaks for itself). But, if you’re going to use Affinity Photo you really should spend the time figuring out Masks. Take your time; it is admittedly a bit of a mind-bender early on. But it opens up so many possibilities that it’s well worth the effort.

Hi!  Is there anyway I can save a custom page preset as a template?  For example, Facebook Page Cover template, Instagram Post template, Business Cards template, etc. with different sizes for each template.  I'm currently using Affinity Designer desktop.  I've tried to find answers and had no luck.  Did find this but it did not have the solution:
Please let me know - this would be such a huge help!