Courtesy to Twitter user XdanielArt (date of publication: 8 June 2024)
Here's the main one I used to get started.
I've started one on creating abstract art in Affinity that seems pretty good and I have a few others in my learning queue from him on specific parts of affinity that look really good but I haven't done yet.
Him and Lindsey Marsh have a bunch of content out there on the whole suite. I took Lindsey's course on graphic design theory that was pretty good but that course used adobe so i used it more for general concepts. I think either of them would be a good place to start.