Manually Create PBR textures in Affinity?

Hi. I make game assets and would like to make PBR textures manually. I do have Substance Painter but found myself wanting to understand the basics from a more hands on approach since Substance Painter is not capable of combining maps at export. I do understand the basics since I been using 3D render programs for years. I have yet to understand fully how to place an alpha map within a base color map using channels for example. So it's mainly understanding how to place greyscale images into the channels that I'm asking about. Any advice is welcome. Thanks in advance.

Quote from NotMyFault

Use Che channels panel.

Assuming you have an RGB8 document.

1. Place the greyscale image into the file. Rasterize the layer.
2. Use channels panel to crate a spare channel from red channel (any of RGB will do, all have identical values)
3. Make the layer invisible (uncheck visible)
4. Select the target layer.
5. Use the channels panel to load the spare channel into the target channel of the target layer.

Thanks! A friend helped me out with that method. Took me a while to understand what I was doing but got it sorted, Now I have to keep doing it so I don't forget. I wish we could just drag and drop.